They already do... on the 3DS. You can buy NES, SNES, Gameboy games, as well as the occasional classic Sega port. If you were lucky enough to get an Ambassador 3DS you also have about 10 GBA games. On top of that it can play any DS or 3DS cartridge so there's a load of content available for it.
Why won't Apple let me install iOS on the Galaxy S7?
I know they sell old games on their own devices, I own several of them (games and devices!). You are missing the point. People have their phone or tablet with them in situations they might not have a 3DS (and certainly not a Wii U!), and lots of people who own a smartphone or tablet may have never owned Nintendo hardware of any description.
IMHO They are limiting both their potential market and their potential profits by reserving old titles only to their own hardware. I really don't think many people are buying a 3DS or WiiU primarily so they can buy old games, it's just a nice bonus for people who are already fans enough to buy the hardware (and probably owned older Nintendo hardware too).
Obviously they would not want to cannibalise current-gen titles, but for older games I think it's a missed opportunity.
I find the question about iOS on a Galaxy silly. It's a very different situation, for lots of reasons. A 3DS or a WiiU is not a smartphone for a start. And surely the analogy should be about what were originally iOS *apps* on Android, which of course there are many, many examples of.
A better analogy might be how Apple put iTunes on both the Mac and Windows, and became fairly dominant, as well as the apparent 'halo effect' of iTunes/iPods/iPhones in encouraging people who had never bought a mac into considering doing so next time they bought a computer. And Nintendo makes games machines.
This looks to me like it could have more of an anti-halo effect. Instead of showcasing an excellent range of classic games for potential Nintendo hardware customers, do they think a silly Mii app will make people want to go and play the most up to date versions Mario Kart or Star Fox or Zelda etc? Really?
I say all this as a fan of Nintendo games, which when done right are some of the best around. And it's a shame more people won't ever play them.