I found an interesting interview with Harry Thompson on the making of Monkey Dust, BBC3's animated sketch show. It ends with this:
Thompson has some cautionary words about selling animated series: “the hardest thing in the world is to get the ear of a controller. It’s hard enough if you’re a producer; if you’re an animator, it’s practically impossible."
So that's encouraging. Then again, he also says:
When we started we weren’t really thinking correctly about the opportunities afforded by animation. We were writing sketches about people sitting around talking, but someone who knew about animation pointed out that an exploding spacecraft would be a hell of a lot easier to animate than a conversation. For a start, the lip-syncing on speech can be quite tricky.
I think I can call myself someone who knows (a bit) about animation, and I have to say that this is bollocks. Perhaps he means that really subtle acting is difficult to animate, which is true - but I suspect that what's really on display here is the preference of many animators; given the choice between drawing a cool spaceship blowing up, and performing tedious lip-sync duties on a couple of people in a pub, a lot of animators would prefer the sci-fi.
When I got into animation I was all for that "Anything Is Possible!" attitude, but the fact is that if you do head into fantastical territory the process does take longer, and is therefore more expensive. There are more design decisions to be made, more special effects to be incorporated (try drawing a realistic-looking fireball, or a water splash - you could lip-sync an entire scene in the time it takes to animate an explosion); and in the end, if you're making comedy, the only thing that really matters is whether or not it's funny.