I've been wondering about this for years. Here's one fanon explonation.
Chaos gives Dracula immense power, but his curse confines him to stay inside the castle. The further he gets away from the castle and longer he stays, the weaker he gets. Think of the Castle as coffin of some vampire lore. I think essentially Drac got duped somehow or accepted this fate, because all he cared/cares is/was killing humans and cursing God. He's the lord of the castle, but he depends on its presence.
All in all, Drac is not and has never been the real big bad. He's a (willing?) puppet of chaos and, ironically, human wickedness.