Usually the mega-dungeon is tuned harder than regular Mythic dungeons, especially considering the hard-modes. (Loot is better as well tho). It wouldn’t make sense to release the m+ version, if it’s already harder and drops better loot.
For not releasing heroic immediately, it’s most likely because to get some more exclusivity.
Well, what about them releasing some exclusive non-mythic stuff then? Why doesn’t that happen? A normal difficulty mega dungeon. With a mythic being released 6-9 months later. Why is that not a thing? It’s unfair.