From what I've played I'm really not sure that's true. I don't think they would have much more success on Wii or DS than on 360. The games just don't seem to have the charm and quality of previous Rare games. If they were constantly releasing cutting edge material, what's made them release average products now? I don't doubt they're profitable, but why have they been relegated to being a pet project of Microsoft's?
If you look at their previous games you can see quite a range of games and a varied portfolio of sucessful games (both critically and sales-wise). After joining Microsoft they haven't produced anything with the critical and commercial success experienced before. Microsoft's takeover of Rare should be held up to the rest of the industry as how not to do a corporate buy-out.
To be blunt, Rare is competing with high quality developers on the 360. Their games are easily over shadowed by high quality games being released all the time. The same games they've released on the 360 would get a lot more attention on the Wii because they appeal to a younger audience and the Wii has a very weak library aside from 1st party titles. Their games weren't good on the N64 either, there just wasn't much to choose from on the N64.
In a nutsehll, Rare has huge competition on the 360 and they would have little competition on the Wii.
Edit: You should also bear in mind that expectations were much lower in previous generations and are still pretty low on the Wii. Its not that their games have gotten worse, they just aren't up to par with high quality Xbox 360 and PS3 games of today.
Rage 2 for X1X (4/5) - Rage for 360 (3/5) - Streets of Rage 4 for X1/PC (4/5) - Gears 5 for X1X (5/5) - Mortal Kombat 11 for X1X (5/5) - Doom 64 for N64 (emulator) (3/5) - Crackdown 3 for X1S/X1X (4/5) - Infinity Blade III - for iPad 4 (3/5) - Infinity Blade II - for iPad 4 (4/5) - Infinity Blade - for iPad 4 (4/5) - Wolfenstein: The Old Blood for X1 (3/5) - Assassin's Creed: Origins for X1 (3/5) - Uncharted: Lost Legacy for PS4 (4/5) - EA UFC 3 for X1 (4/5) - Doom for X1 (4/5) - Titanfall 2 for X1 (4/5) - Super Mario 3D World for Wii U (4/5) - South Park: The Stick of Truth for X1 BC (4/5) - Call of Duty: WWII for X1 (4/5) -Wolfenstein II for X1 - (4/5) - Dead or Alive: Dimensions for 3DS (4/5) - Marvel vs Capcom: Infinite for X1 (3/5) - Halo Wars 2 for X1/PC (4/5) - Halo Wars: DE for X1 (4/5) - Tekken 7 for X1 (4/5) - Injustice 2 for X1 (4/5) - Yakuza 5 for PS3 (3/5) - Battlefield 1 (Campaign) for X1 (3/5) - Assassin's Creed: Syndicate for X1 (4/5) - Call of Duty: Infinite Warfare for X1 (4/5) - Call of Duty: MW Remastered for X1 (4/5) - Donkey Kong Country Returns for 3DS (4/5) - Forza Horizon 3 for X1 (5/5)