Emphasis on might. Remember - that’s only because of his formerly unique status as the THE Uncorrupted Black Dragon. There are more now - and-
SPOILERS
Much more qualified ones. Sabellian is a black dragon who’s seen more than his fair share of war and conflict - and is 100% better qualified to lead the flight.
At the moment, all Wrathion is, is an arrogant child imagining himself to be equal and about to play with cosmic forces.