Because he directed 300 a few years later which was a success and a semi-critical baby (and very entertaining -- not good -- but very entertaining). But it wasn't taken seriously enough (yes, I think Znyder wanted 300 to be regarded as a big, dark war drama), and thus he directed Watchmen. Watchmen wasn't a success and a critical darling, but for some reason WB keeps pushing him doing those dark and grim and boring CB-movies. Is it because of Nolan? Yes, probably.
I think it would be very interesting to see a comedy directed by him. If he did a romantic comedy it would be his best picture.