Because its more accessible to play a 2D simplified platformer game than a 3D game that requires some complex controls and physics (you're defying gravity after all) to get into. NSB isn't so much Old School as it is simplified back down to resemble the other 2D Mario games. There's nothing wrong with that and as you can see it works out very well, but its not the same thing as expecting Square Enix to produce another 16bit FF game.
As far as the Sonic games - wow. Don't look into that as an example of what fans want. Fandom is crazy. There is some wild faux-psychology going on in the minds of "hardcore" fans of series. They develop strong emotional bonds to a certain thing that happens to fulfill them (usually during childhood or adolescence) to the point of idolization, then go down a bifurcated road where they want more and either A. cannot get any more, thus leaving the fan unfulfilled or B. get more until sooner or later they get too much of a good thing. The response to the end of the same recognizable positive stimulus is to blame everything about the game for that cardinal sin except your ability to recognize that you already got your fill and don't want to accept it.
Things change. Do you have any idea how lucky you are that you got 6 games in a row that all produced that same positive stimulus you enjoyed? You can't just turn that on when you're developing a game. Square-Enix hasn't just decided to make sucky games people like you can't (or rather refuse to) appreciate, they decided to do something different. Now you're bitching about games you haven't even played, games that are the closest things you're going to find to what you want that somehow still aren't good enough, and its Square-Enix's fault?
I'm not a doctor so all that should be taken with a grain of salt, but I mean c'mon now. How hard can it be to wake up to reality and just enjoy something for what it is, not what it used to be? Videogaming is supposed to be fun, and you're denying yourself that.