I wonder why...? Why couldn't birds get as large as the azhdarchids?
My personal theory about this is that maximum wingspan in birds was
constrained by the fusion of the carpometacarpus, which required the
elongation of primary flight feathers for the development of thrust.
Pterosaurs had no such hereditary mechanical constraint and were able to
extend wingspan by elongating the fourth finger. This was probably a
stronger mechanical arrangement for the outer part of the wing than what
evolved in birds, which at extreme sizes allowed for longer wings that could
develop more thrust to support more massive bodies.