Not that hard to guess tbh. We still haven't gotten Clampearl line Keckleon, and Smeargle. Quite a few of the remaining Gen4 mons listed up have a gimmick like the latter three mentioned.
And of course normal families like Gible, and Hippopotas are probably being dragged out for the reasons you mentioned. If we try to be optimistic and say that Niantic isn't dragging the rest out just for the sake of prolonging the release, maybe they're planning some sort of Sinnoh event and plan on releasing them there? Idk. Honestly, I've gotten use to the slow release. It's actually not that bad, but I really do wonder about the true reasoning behind this release.