More like it's a difference of complexity and resources between the 2D mario games and the 3D ones. They could probably pump out 2-3 2D mario games with the same size team in the same amount of time as it takes to make a single 3D mario game.

So why haven't they? Why are there 0 original Gameboy Color, and Gameboy Advance 2D Mario games???

Wasn't Yoshi's Island Super Mario World 2?

I would love 2 mario games for the Wii. Would it get old? How many final fantasy's were there for the SNES? How many for the PS1? How many for the PS2? Did it get old? 

Nintendo Wii SHIRO!

The NES lasted longer than the other systems (7 years), which I think is the biggest reason why it got two real Mario games. (SMB2 in Japan was, I understand, more like SMB1 Expansion Pack; SMB2 for us was, of course, a drug trip.)

So unless the Wii lasts way longer than the 5 years the SNES, N64, and Gamecube all did, I am going with "No." But hey, maybe if the Wii crushes its competition it will last longer despite the then-ancient hardware. In that case I would say "Probably yes."

Look at it this way: SMB3 launched in Japan in 1988, 5 years after the NES came out. If the NES had been replaced in the same timeframe as the later systems were, that game would have been the next system's launch title instead.

P.S. a.l.e.x59: 4 Mario NES games? Do you mean SMB, jSMB2, aSMB2, SMB3?

Actually, the Nintendo had three real Super Mario Bros. games. Actually, the Japanese game rip off still kind of counts as a real Mario game, since both games were made by Miyamoto. Does Super Mario Bros.: The Lost Levels not count, if Super Mario Bros. 2 doesn't? Technically, all four are real, but if you don't count Super Mario Bros. 2: U.S.A., than Super Mario Bros. 2: Japan will back it up as a real Super Mario Bros. game. As it stands, there are four Mario platformers. Anyways, Super Mario World 2 is not a Mario platformer because you control Yoshi, not Mario. In fact, only baby Mario is in the game. It's more of a Yoshi Story prequel. So the Super Nintendo really only has Super Mario World. As long as you control Mario in a platformer, you are playing a Mario platformer. If Wii defeats the Playstation 2, we might get 3 Mario platformers. I mean, it has only been more than half a year since the Wii has been out, and Super Mario Galaxy is coming soon. Do you really think that if the Wii lasts for 8 years, that no new Mario game will come out after Super Mario Galaxy? Will we have a blank period of 7 years, or more, before a new Mario game comes out? After Brawl, Kart, and Galaxy come out, the Wii will live on for many years to come. Soon, Brawl, Kart, and Galaxy will get old, like Sunshine, Dash, and Melee. Now, if they get old before the Wii dies out, they will need to get replaced. If Nintendo stops making other games that no one wants to play, they will have more time to make more games that we want to play, like more Mario games. I don't think we can ever have too much Mario (not including Mario Party. Damn it, stop it already!). I'm sure the Wii will last long enough for more Mario games to come out. But something tells me that there will only be one of each Mario franchise (excluding Mario Party) because that's just the way Nintendo does it. It's almost like a law. One Mario game per console, just like they have to make a crappy movie game for every animated movie that comes out, and new sports games, of the same sports, every year. Only time will tell.

I could see a second 3D Mario platformer on the Wii, but honestly, what I think we'll be seeing is a new Mario platformer trilogy on the DS.

 I predict a New Super Mario Bros. 2 will come out soon and maybe even a New Super Mario Bros. 3 in due time.

A lot of good replies here. I am most in favor of a New SideScrolling Mario Game for Wii, that would make me extremely happy. I am starting a campaign on my website for it, if that can tell you how much I want one.

But, I think DarthDevidium hit the nail on the head. I think Nintnendo needs a new "hard core" IP, that is at the Mario-Zelda-Metroid level.

It can be cute, it could be cool, it could have space marines for all I care. They have so much talent, I am sure they can come up with something.

i agree with a new sidescroller on Wii -- possibly on Wiiware?

If not, then i definitely agree with a New Super Mario Bros. 2 and 3, just without as many power ups

I was speaking similar things yesterday to a mate of mine, how it would be nice to see another Mario 3D platformer on the DS, or another Mario Kart, 1 per gen isn't enough anymore!

There sure won't. Part of what makes these games (Mario, Smash Bros, Mario Kart) so successful is the anticipation that builds over the years.
Oh how I hoped they would make another Mario Kart for the Game Cube... I absolutely love Double Dash, but by now I could probably play it blindfolded :)