Nintendo made the WII U backwards compatible with the Wii and gue# what? It still wasn't selling like crazy because of it. It didn't set the world on fire, it wasn't a major reason why it was even selling and majority of the games people play on it are newer games.
Yes backwards compatibility is awesome as an option to have, but when an overwhelming majority don't use it an a very small minority base uses it, financially it makes no sense. They can't just spend crazy money on the feature, do all the same talks with publishers again to allow it and just be like here you go and no one uses it. It's a waste of time for them and it's still great that Microsoft has done it
But it's clear they only did it to convert what isn't there and this wouldn't be the case if they had the user base needed for something like this to see the light of day.