That doesn't help or answer the question.I'm not an evolutionary scientist, but I can help you anyway. You're asking the question incorrectly. The question is, what could stop it if it's possible and the adaptation yields a competitive advantage?
There's no food in the sky, there's no shelter in the sky - it's all on the ground.
What purpose would it serve a creature to evolve the ability to fly when it's existence relies on what's on the ground?
It's ancestors?Why wouldn't it? It's ancestors were terrestrial, i.e., walked on land. Whales have pelvises for the same reason that you have a tailbone. It's a vestigial structure.
Oh ya, the fish grew legs and came onto land, they also grew the right eyes, skin & internal organs at the same time (including a pelvic bone).
What, some of them decided life was better in the water and went back?
Then why not evolve gills from their lungs? Why not lose the pelvic bone?