I’m not saying it was impossible, but it clearly wasn’t as simple as most people assume it should be, and just because it’s possible to get it working via hacks doesn’t mean it would be up to Nintendo’s standards for a paid release. That’s likely why they never released GBA games for sale on 3DS, either—yes, it was possible, but due to it relying on backwards compatibility, certain key functions like sleep mode aren’t supported, and I assume Nintendo didn’t want to release games like that for sale (and as such only ever released them for free to early adopters).
Also, regarding downloadable Wii games on Wii U, that wasn’t possible at first, either, until a few years later when it only became possible thanks to
NERD’s hard work:
Nintendo even made a big deal of this in a Nintendo Direct. So, no, these things aren’t as simple as many people think they should be.