By norm, there is no clear reason for this. Most Kalos Pokemon have a very colourful movepool, learning nearly every move they could possibly learn by breeding with other Pokemon. Considering the fact that limited Pokemon could breed with each other in Kalos and passover a logical move to their offspring ( for example: Delphox and Chesnaught being in the same eggroup but being pure parallels type-wise), it seems rather impossible in the region itself. Most Pokemon naturally learn support/status moves, either through level-up or through TM. The very reason many people dislike many Kalos Pokemon and only love a few, the few being offensive. Due to almost all possible status/support moves being accessible, only a few get to gain access to more of a little group of moves.
That being said, we all wish for more things in Pokemon, and despite GF knowing these needs, they know they can't always add them. Odd be true is because they actually do care about a certain amount of balance within the games.
In all truth, we don't know why GF decided to gave limited eggmoves to the Kalos Pokemon or even made them mediocre to what we usually expect. You could really say that only GF knows the answer.
