The main reasons are a combination of weak first party output and the console being built on the false premise that third party multiplatform games are important. After the Nintendo 64 had failed, Nintendo's main takeaway from it was that it is about third party software, so the GC was designed accordingly. It wasn't until a generation later that Nintendo recognized that it is about Nintendo themselves and what they stand for, so the Wii was designed accordingly. Then they learned nothing from the Wii and made it about third party multiplatform games again with the Wii U, a console that failed even harder than the GC. Then we got Switch which has the distinct Nintendo identity again. There's a clear pattern of success and failure, including the NES and SNES which weren't about third party multiplatform games either.

The main reasons are a combination of weak first party output and the console being built on the false premise that third party multiplatform games are important. After the Nintendo 64 had failed, Nintendo's main takeaway from it was that it is about third party software, so the GC was designed accordingly. It wasn't until a generation later that Nintendo recognized that it is about Nintendo themselves and what they stand for, so the Wii was designed accordingly. Then they learned nothing from the Wii and made it about third party multiplatform games again with the Wii U, a console that failed even harder than the GC. Then we got Switch which has the distinct Nintendo identity again. There's a clear pattern of success and failure, including the NES and SNES which weren't about third party multiplatform games either.

Nowadays there's the widespread belief that Sony and Microsoft make traditional (or conventional) consoles while Nintendo is the odd one out. This belief has it backwards, because it's Nintendo who makes the traditional console. Sony and Microsoft consoles are about playing PC games on a device that is easier to use than a PC. When you think of the classic video game consoles made by Atari, Nintendo and Sega, are PC games the first thing that comes to your mind? Of course not, because the traditional console isn't about playing PC games. The essence of the traditional console is bringing the arcade experience into people's homes, and the arcade experience differed from the PC experience in that all the games were standing next to each other and had to be instantly good, otherwise potential customers would just walk over to the next arcade cabinet. This is what led to the "easy to learn, hard to master" mantra.

Consoles did evolve over the first few generations, from housing mostly ports of arcade games to eventually being more about games specifically developed for consoles. But said games followed the arcade mantra of "easy to learn, hard to master" which is why they maintained their addictive essence. Whenever you come across a game that shoves a lot of tutorials down your throat, it's easy to realize that said game isn't what it should be about. When you think about it like that, you can put two and two together why third party multiplats are at odds with what the market expects from a Nintendo console and why those multiplats barely move the needle on hardware sales for Nintendo.

The Nintendo 64 focused on 3D games, and early 3D games feel clunky and stiff in comparison to the brilliant fluidity of the gameplay in the 16-bit era. Super Mario 64 may have been the foundation for a lot of 3D gaming, but it pales in comparison to how good SNES games were. Nintendo's direction with first party software did more damage to the Nintendo 64 than the loss of third party IPs to Sony's PS1. Nintendo fans skipped the Nintendo 64 because Nintendo wasn't making the right games anymore. Then with the GC Nintendo didn't even cater to the expectations of the fans of the 3D games anymore (most notably, Super Mario Sunshine and The Wind Waker were controversial), so many of those fans skipped the GC. Nintendo went from bad to worse.

The GC had around 600 games, up from the around 400 of the Nintendo 64, but third party software mattered little for sales because most of those games could be bought elsewhere on top of coming first and foremost from American and European publishers whose roots are largely in PC gaming.

I would strongly disagree with that. The only thing that was conducive to third-party support on the GameCube were the specs. The GCN was the second-most capable console of the sixth generation. And the N64 had been the most capable of the fifth. This was the last generation Nintendo was competitive in specs for home consoles.

The processing power, RAM, etc. were good for third-party developers. But Nintendo threw tons of hurdles in their way. Namely, Mini DVDs that crippled the size of games. Though admittedly, it was not as bad as using cartridges for the N64. Someone else said on this thread that the average PS2 game was about 3 GB. That's 2 GameCube discs. And most developers didn't want to deal with 2 discs, or they had to compress the game to fit it on one or two discs.

The GameCube controller was also not conducive to third-party games. There was only one bumper, no dedicated select button, an oversized A button/undersized X, B, Y buttons, and an undersized second analog stick.

And online on GameCube was virtually non-existent. That was a big slap in the face to third-party developers, especially as online console gaming was really picking up around 2002-2005.

Nintendo learned very few lessons from the N64 to GCN on designing their consoles for third-parties to take advantage of.

N64 was powerful but difficult to work on aside from cart space. I don't have the time to dig up old magazines but I do know Miyamoto apologized publicly for N64's difficulty to work on and Gamecube was made to make it easier on 3rd parties. Thing is, Nintendo is still Nintendo in being clueless at times. So they had small discs. An odd controller and while Dreamcast,Xbox opted for Direct X. PS2 with OpenGL. Nintendo made their own API. GX, which they used all the way up to Wii U. Switch is the first console I know of from Nintendo that uses a standard API.

You must have forgotten the huge backlash it got when Nintendo revealed it. It alientated a fair portion of the fanbase with the artstyle. Its easy to think of 3D Zelda as this ever morphing visual entity at this point but back in 2001, all 3D Zelda games had been suedo realistic and dark/surrealist. Suddenly you have "toon link" being presented as the future of the franchise. WW sales on Gamecube speak for itself. 

You should address what I said. It doesn't matter if the Nintendo 64 had revolutionary games when it's abundantly clear in hindsight that the Nintendo 64's best games aren't on the same level as the likes of Super Mario World, Donkey Kong Country trilogy, A Link to the Past and Super Metroid. Game quality was higher on the SNES than on the Nintendo 64.

Something that cannot be mentioned often enough is that the biggest factor in the shift of third party support in the fifth generation was not cartridges vs. CDs, but no moneyhats vs. aggressive moneyhats. If the Nintendo 64 had used CDs, third party games would have gone to the PS1 all the same because Sony was paying for them.

People keep having this habit of sticking up for bad third party developers. Whenever a third party pulls off some bullshit due to incompetence or laziness, way too many people choose to blame Nintendo for the result rather than the companies that actually made the games. There's real world data that it was doable for third parties to make high quality GC games, because there were third parties who could do it.

That's why the storage capacity of the GC's discs can't be an excuse. That's why the amount of buttons on the GC controller wasn't a big issue because most games didn't need so many buttons to begin with. Online gaming wasn't a big deal during the sixth generation either.

What you are doing here is the equivalent of sticking up for WWE 2k18 on Switch by coming up with all possible excuses.

"The game has a partial download because Nintendo chose an expensive storage medium." - Somehow not a problem for The Witcher 3.

When you judge a console, you do so by its best games, not its worst. It's bad enough that third parties have a long history of pulling off bullshit on Nintendo consoles, but it's even worse for gamers to defend said bullshit when there's actual evidence of what was really possible.

Nobody knew what the fuck a "third party game" was in the NES era, lol. That wasn't a thing. All games were just "Nintendo games" and even all video games were just called "Nintendo" even if you were playing a Tiger handheld or something.

It's also due to the market being more limited with fewer studios and development being more restricted (remember in America NES was released after console gaming crash), fewer quality titles came out from 3rd party during NES/Master System era