GF saves certain megas until a game needs them in it's story. Like Metagross and Steven, Salamence and Zinnia/Drake, Gallade and Wally.

Whats a Gen 1-4 mega evolution people really want that hasn't happened yet? Story importance in Z is likely the reason we haven't gotten some of those yet. They wanted Flygon in XY but they couldn't make a good design yet so exclude that. Flygon could still happen in Z if they got the design.