His acting is often just terrible. They developed 7 and the Doctor and I'm pretty sure their sparkling performances had a lot to do with it. Why would anyone want to write gripping dialog for a guy who can't even show some expression when he finds out his lover has betrayed him?
And this is the eleventh episode of season 1! So the theory that poor Beltran was sooo bored after years of being neglected that floats about doesn't bear up.