Richard Coyle himself said in an interview that he was afraid of being typecast:
I was very keen that that character didn’t stick with me. It’s the kind of character that does.
I’m an actor and I want to be an actor when I’m 60. It’s a lifelong process, why cut it off by boxing yourself into a little pigeon hole early on?
It was a very difficult decision; incredibly difficult because there was something nice about the fact that I knew, for three years, once a year we all came together for three months and had an almighty laugh.
The writer/creator of the series, Steven Moffat, had this to say about Richard Coyle's departure:
Richard didn’t want to return. Or do a goodbye episode. Or in fact talk to me. Hurtful behaviour, but it’s showbiz, these things happen. Can’t give you any insight, except these wise words, spoken by – er – a wise person: an actor will forgive you for anything but their first success.