Erethian said:
Partly because it's a bridge game that is designed to introduce those more comfortable with 2D Mario to 3D Mario elements, and partly because the software ecosystem for 3DS will be a lot healthier than it ever was for Wii.
Not to say that the 3DS won't have a healthier software ecosystem than the Wii (because based on 3rd-party support, it sure looks like it has a very good chance of doing just that), but let's not sell the Wii short.
The Wii was definitely disappointing for mid-range titles, but it has been a beast when it comes to the heavy hitters. For a quick comparison of million-sellers (Famitsu):
PS2:
3,555,469 - Dragon Quest VIII: Journey of the Cursed King
2,325,215 - Final Fantasy X
2,322,541 - Final Fantasy XII
1,960,937 - Final Fantasy X-2
1,614,806 - Dragon Quest V: Tenkuu no Hanayome
1,438,743 - Gran Turismo 3: A-Spec
1,197,349 - Dynasty Warriors 4
1,129,196 - Kingdom Hearts II
1,116,786 - World Soccer Winning Eleven 6: International
1,100,238 - World Soccer Winning Eleven 7
1,083,079 - Hot Shots Golf 4
1,066,749 - Gran Turismo 4
1,059,585 - World Soccer Winning Eleven 8
1,050,319 - World Soccer Winning Eleven 10
1,042,819 - Dynasty Warriors 3
1,024,338 - Samurai Warriors
1,003,874 - Onimusha 2: Samurai's Destiny
Total - 25,092,043 - (17 titles)
Wii:
4,264,980 - New Super Mario Bros. Wii
3,688,089 - Wii Sports
3,561,787 - Wii Fit
3,255,433 - Mario Kart Wii
2,801,209 - Wii Play
2,511,745 - Wii Sports Resort
2,278,075 - Wii Fit Plus
2,094,464 - Super Smash Bros. Brawl
1,922,243 - Wii Party
1,427,575 - Mario Party 8
1,221,459 - Animal Crossing: City Folk
1,070,743 - Monster Hunter 3
1,005,649 - Super Mario Galaxy
981,606 - Super Mario Galaxy 2
928,495 - Donkey Kong Country Returns
907,975 - Super Mario All-Stars
?,???,??? - Dragon Quest X
Total - 33,921,527 (+ DQX) - (16 titles (+ DQX))
SMG2 and DKCR will certainly top a million. The only questionable one here is Mario All-Stars. But Media Create has it 50k higher than Famitsu, so if Nintendo decides to ship some more for the upcoming holidays then it may squeeze past a million. But it may very well fall just a bit short.
In any case, this shows that the Wii was extremely competitive with the PS2 when it comes to the top-selling titles. And Mario falls into that category. (and I suspect this helped Square Enix feel comfortable about keeping DQX on the Wii all this time).