By that reasoning, nobody who was born after 1995 should be playing those games.

Why is 3D Mario not as popular as 2D Mario?

"It's because 3D Mario is too difficult, too complex, too sophisticated. It's for the core. 2D Mario players are casuals."

The above is a commonly expressed premise that is also shared by Nintendo. It's the reason why Super Mario Galaxy 2 came with an instructional DVD that explained in great lengths how to move an analog stick, how to perform a jump etc. It has probably been uploaded to Youtube, so you can watch it, if you are interested. And yes, it's real.

But anyway, let's tackle the question at hand. What is it that makes 2D Mario a more popular video game? Or in broader terms, what is a game anyway? A game at its most basic definition is an activity with a set of rules that leads to an outcome of winning or losing. Draws can also be possible, but most games usually implement some sorts of tie-breakers to create a win or lose scenario. This definition encompasses board games, card games, sports and, of course, video games. For Mario platformers specifically, winning means to reach the end of the level. Losing means to fail at that goal by various means, like getting hit by enemies or falling into pits.

Let's take a look at what initial impression Mario platformers convey by counting the amount of real chances to lose the game in the opening stage. The first level usually sets the tone for what is to come, so we don't have to go any deeper into it. Examples are restricted to three games each, because it's a big enough sample and the omitted games don't change the outcome. In every case, a straightforward playthrough is assumed. In other words, the most obvious path. So no pipes, shortcuts or the like. Deadly pits are holes you have to jump over in order to complete the level; enemy confrontations are the enemies you have to deal with, meaning they cannot be simply ignored and thus have to be defeated or jumped over.

*There is an area where you have to collect star pieces and can fall off, but that's not the same as having to jump over a pit.
**Two enemies hold keys and they are the only ones that have to be dealt with, but we are talking first time playthrough where it's inevitable that more than two enemies will have to be confronted.

As if it isn't clear enough already that 2D Mario games present a higher threat level, two other things need to be noted:

1) 2D Mario levels are shorter, increasing the threat level even more.
2) In 2D Mario games, the player dies upon taking one hit, unless a power-up has been picked up beforehand. 3D Mario games come with life meters, so the player has to take eight hits in order to die (in 64 and Sunshine) or three (in Galaxy 1 and 2).

Another thing that needs to be noted is that Super Mario 3D Land and Super Mario 3D World try to be more like 2D Mario games than previous 3D iterations. Regardless, the fundamental aspect that most enemies can be safely ignored persists. Also, the player starts as Super Mario, so it's two hits until death, thus more forgiving.

In summary, the chance to lose in 2D Mario is considerably higher than in 3D Mario. It's indisputable. What this means is that the initial premise is turned on its head: The 2D Mario player is the gamer who seeks the thrill, the challenge, the omnipresent threat of losing the game. The 3D Mario player is the casual who wants to wander off minding his own business. As such, 3D Mario fails to reach the same popularity as 2D Mario because it is perceived as boring, lame, tedious. And the three-dimensional environment will forever prevent it from becoming as popular as 2D Mario by default. Nintendo's attempts to give all the love to 3D Mario over 2D Mario won't change that, rather it highlights how futile their attempts really are.

The table isn't flawed. I don't even understand what counterpoint you are trying to make.

As for the rest, you are seriously deluded. The Nintendo console that had Zelda, Metroid and 3D Mario, but no 2D Mario, was the worst-selling one. In fact, when you look at Nintendo's home console history, the systems that had the biggest amount of simple games sold the best. So logically, one has to conclude that it's not the complex games that are the biggest hardware movers.

NSMBU failed to move more hardware, because the Wii U itself is a piece of trash. Just give it a bit of time and see one supposed big system seller after another fail at doing its job.

You mean enemies? Which I accounted for?

I have no problem to concede that The Wind Waker and Sunshine weren't the games that people wanted. But let's say the GC Zelda and 3D Mario had been up to standard, sold as much as their N64 counterparts and each of those additional software sales would have equated one additional hardware sale. The GC would have still sold fewer units than the N64.

I cannot believe users have not realized how flawed that table is. In game A you face blue aliens, in game A 2 you face red aliens since in game A 2 there are no blue aliens there is no threat (what logic is that?).

Also the reason why 2d mario would sell more than 3d mario because it is more straightfoward (which has already been stated many times in this thread) but it is also little more than that.

The thing is the games that sell nintendo hardware are the more complex games such Zelda, Metriod and 3d Mario (lets just put it in there for good measure). While these by themselves don't seem to do mindblowing numbers the truth is any Zelda, Metriod or 3d mario fan would drop money on a new console to play these games (provided the games are up to standards). But the tricky part is that a Zelda fan may not buy a console for Metriod, and a Metriod fan may not buy a console for 3d mario and vice versa because each is complex and different in thier own way.

While on the other hand because of 2d mario simplicity  it would be bought by mostly any gamer once it is on their console. So what you end up with is a zelda fan buying a nintendo console and picks up 2d mario btw, a metriod fan buying a nintendo console and gets 2d mario btw and a 3d mario fan picking up 2d mario btw, increasing the sales of 2d mario by 3x.

Which is the reason why NSMBU failed to move as much hardware but will probably outsell any of these games IF they make it to wiiU.

Oh right, there were also waterblob-like cannon shots falling from the sky during one area on the path to the mountain. Doesn't really make a difference though.

Those reasons are nonsensical though. You assume that there is no overlap between Metroid, Zelda and 3D Mario, but 100% overlap between 2D Mario and each of the three aforementioned individual games. Nevermind that NSMB Wii outsold Metroid, Zelda and 3D Mario combined on the Wii. Or that NSMB did the same on the DS. Why the denial that 2D Mario is a system seller?