It's just what Game Freak wanted, but I have a kind-of theory :3
Druddigon weighs 306.4 lbs (139.0 kg) and is 5'3" (1.6 m) tall. A big Pokemon, no? And if you take a look at its wings they look rather small and almost paper thin. They don't look strong enough IMO to lift it up for long periods of time.
And the Pokedex entry for Black says:
> It warms its body by absorbing sunlight with its wings. When its body temperature falls, it can no longer move.
So it seems it's wing are necessary to it's survival, as if it's body temp falls, it can't move.
Hope I helped! :)