this is just a guess, but it's probably for game balance.That makes sense. Is there any reason for VGC doing that?
certain moves are no longer legal on certain pokemon, like heat wave zapdos, drain punch mewtwo, and quite a lot more
they picked and chose certain things to be egg moves in gen 6, like defog scizor, stealth rock skarmory, etc.
i don't think anyone from nintendo or gamefreak have said one way or the other why