User blog comment:Kogath/Monster Hunter Generations Discussion/@comment-26532315-20160304134118/@comment-3298463-20160304142638

I don't think Diablos is meant to be a staple of the series like Rathian and Rathalos, so it doesn't have to be in every game. It was just a monster they decided to keep in Tri because (I guess) its mechanics worked well enough without needing a replacement.

I mean before Tri it was pretty clear that the Raths were the "mascot" monsters, but Diablos was just another wyvern.