Because Game Freak does what Game Freak wants.
Honestly, we have no idea why GF does what they do. There's a phrase called "Game Freak logic" that is used to answer this sort of thing. It's really the only explanation.
If it were up to me, I would have made all of the Mega Stones flow nicely, not just jam -inite at the end of the Pokémon's name. Here's what my version of some of the weirder Mega Stones are:
These are the only ones I rename, because I think they're original names are ugly (Glalitite? How do you derive that from Glalie?).