NSMB as a series hasnt been that hard either. Not even close to the older Marios, but maybe thats more of a reflection of the times than anything.

I'm surprised at how many people agree with your OP considering the general consensus is that 2D Mario is easier. Which is, of course, unavoidably false. I never understood why people ever said that. It seems so oblivious that it makes it out to seem like these people have never actually played 2D Mario games...!

But... "As such, 3D Mario fails to reach the same popularity as 2D Mario because it is perceived as boring, lame, tedious." This sentence is quite ridiculous. Haven't almost every 3D Mario game as of late been perceived as adventurous, fun, and exciting?

Super Mario 3D world is pretty easy so far. The only difficulty is judging depth when the camera is presenting the level as 2D Mario in multiplayer. The 45 degree walking angles don't help either in 3D environment.

One thing is for sure: Speed runs are never as satisfying in 3D as 2D Mario.

I'm not going to start that ridiculous "sales don't equal quality" argument, but I think saying what you said about 3D Mario games is definitely harsh. The challenge, replay-ability, and streamlined feel that 2D Mario games present aren't really there, but to say they are lame and boring seems off.

because which game is easier than the other is based on facts? no, some people might find 3d Mario harder and vice versa, i know. crazy, right?

Basically, I think you show it has nothing to do with complexity of the game itself.

I feel it has to do with perceived simplicity in gameplay and nostalgia due to SMB being just a popular game in of itself.

Though I like the 2D games more personally.

I always get confused over which one is the system seller (so do a lot of others in here), that's all I know.

Personally, I find the 3D Mario more appealing, ever since Super Mario 64.

A lot of it with me is because I always had more fun playing the 2D Mario games like Super Mario Brothers 3 and Super Mario World, but I enjoy the 3D games as well.

I've always looked at it from a slightly different perspective, though just a yard from where you're standing.
People of all types tend to gravitate towards games that are more 'arcade'.
3d mario is not arcade, 2d mario is.

As it has been said, it's not difficulty which is the barrier to 3D games, it's complexity. 3D Marios aren't harder, they're just more complex. In particular, you can get lost. There is also the issue of "timing" things like jumps in 3D which is a challenge. This is a most significant issue when it comes to females, who have a scientifically proven difficulty with 3D visualization. If a large portion of your potential customers have a challenge knowing where they are on a given level, you have less potential customers. 3D world helps that a little with the semi-locked camera.