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. 

I think one of the big problems is it didn’t actually come off as a Nintendo console. It was more like a stripped down PS2 clonebox for kids.

Nintendo’s handhelds sold fantastically well, and the Wii was incredibly hyped. GameCube lacked the inspiration of those other systems.

But I do think software was a big issue, as GameCube also lacked a killer app.

You kinda had to be there.

It's easy to look back on it now through the lens of nostalgia and wonder why it didn't succeed, but at the time, coming on the heels of the N64 and alongside the Xbox and PS2, Gamecube was basically the definition of "uncool" in everything from the way it looked to the approach many of its core franchises took.

It was a colossal failure to read the room and provide what was seen as appealing and desirable in gaming at the time.

Because of PS2. It caused the failure of the Dreamcast, the Gamecube and didn't help the success of the original Xbox. Why bother buying another console when everybody on Earth (almost) was buying a PS2 ?

GC didn't do well because it tried to be PS2 lite and the era back then third parties could afford to be exclusive to one platform.

No built in dvd player. That's it.

I think the reality is that PS2 was the top dog, and XB was seen as the alternative. There wasn't enough room in the market for GC. The Gamecube looked like a toy, was named like a toy, and was generally not taken seriously.

The two biggest reasons the Gamecube failed were the Playstation 2 and the Nintendo 64.

Nintendo lost all momentum with the N64. The system sold well at launch and for the first half of 1997, but its sales fell off a cliff after that. People say the Wii was frontloaded sales-wise, but nothing can compare to how front-loaded the N64's sales were. And those early sales came from it being a Nintendo console at a time when Nintendo was coming off the SNES and the market leader. The N64, with its more expensive games, long droughts, and small library compared to the Playstation destroyed the allure of a Nintendo console even if it had some of the best games of all time. Nintendo's kiddy image didn't help either. By the time the Gamecube launched it had been 4 years since a Nintendo home console had sold well and people were used to not playing on Nintendo consoles. The Gamecube was an attempt at course correction which came too late to bring back the people the N64 had lost.

The Playstation 2 was also such a mammoth that there was no competing with it. It was following on from the first home console to ever sell 100 million units, had a DVD player, and had all the momentum and a 1-year head start. With no momentum and nothing that stood out about it the Gamecube was an 'also' console to the mass market. That's why Nintendo stopped trying to compete with conventional consoles and has gone for a more unique system ever since.

As with most systems, games.

PS2 had all the great games.

Dreamcast and Gamecube had barely any games, and the Xbox should have failed as hard as the Dreamcast but it had a slew of great games not found on the PS2. I would say the same with Gamecube due to it's Nintendo games, but it needed more clearly.

Lack of games + no DVD player.