Sonic the Hedgehog 4 Episode: II

[ApocHedgie]

I think it just sort of weirds me out that iOS, W7 phone and Android get this
and it's completely absent from Nintendo's products,
especially when Colours being on only Nintendo products sold just as well as Generations which was on all current gen. excluding Wii.

Granted, it's because of hardware restrictions, but I really never thought the 3DS or Wii was that constricting for a side-scrolling Sonic game.

It's because of the 50MB limit for WiiWare titles. As it is Sonic 4 Episode 1 barely made it, and they had to simulate the music with a soundfont to make it fit. There's a possibility of it getting put on a disk later on in a Sonic 4 Complete pack, but there's no guarantee for that either.