Perhaps the Balrog underestimated Gandalf, thinking it could cast him into the abyss, then fly back up to deal with the rest of the Fellowship. Alternatively, maybe it calculated that its best chance of defeating the wizard was to grapple with him at close quarters. Either way, the Balrog falling doesn’t necessarily mean that it wasn’t capable of flight.
Perhaps the Balrog underestimated Gandalf, thinking it could cast him into the abyss, then fly back up to deal with the rest of the Fellowship. Alternatively, maybe it calculated that its best chance of defeating the wizard was to grapple with him at close quarters. Either way, the Balrog falling doesn’t necessarily mean that it wasn’t capable of flight.