It wasn't cancelled per se, rather HBO only ordered a single season and they never renewed the show. Honestly it's astounding that it got one season at all considering what HBO were doing to animated series at the time.

The show is fantastic but as far as I'm aware they didn't pull great view numbers, which can probably be attributed to some less than stellar advertising.

That's true. I only heard of it because I follow Charles Huettner on social media. If it had been a couple of months later, after I'd deleted Instagram, I'd never have known.