Because Gamefreak did a poor job at balancing trainers in XY.
In past games you didn't need to the Exp. Share at all and you could easily level your mons evenly on just trainers while still not being too overleveled.
They designed XY in such a way that they EXPECTED you to use the Exp. Share., but your mons end up overleveled and the game is easy. Solution: Just turn it off? See here's the problem with that, if you do turn the Exp. Share off, then the game gets much harder, because you can't evenly level your mons on trainers anymore like you could in the older games, because Gamefreak scaled the trainers expecting you to use the Exp. Share. As a result you end up being underleveled unless you grind on wild mons in every new area.
TL;DR: With the Exp. Share on, you gain TOO much Exp. With it off, YOU DON'T GAIN ENOUGH. Gamefreak effectively removed "normal" mode. You only have "easy" and "hard".