Upbringing I would think (with Carrot being an official dwarf).
I think it's Angua who's the one with the 'problem' - she never really seems comfortable with her wolf form, in fact she appears to be ashamed of being a werewolf most of the time in some ways. With her family being the way they are too, I think she's afraid of what will happen if she settles down and has a family with someone - anyone, not just Carrot.
On his side I don't think that's so much of a issue, but being a dwarf on the inside they're hardly the most spontaneous or passionate people (I'm talking intellectually and romantically here
) unless it's trolls and even there they can get on with them if they put their minds to it.
In their mountains they do seem very staid and conventional, so from that PoV, with Carrot having the mindset if not the body, he's the sort who doesn't like to rush matters of this nature and needs a long courtship as it were. So he doesn't expect to be married too quickly anyway and of the two he's least worried about 'marrying out' of their culture, so perhaps he's just waiting for Angua to decide.
Ironically I think her wolf personality is more than ready to commit - he is her alpha male and it's definitely her canine/cop loyalty that keeps her with him. Her human side seems to resent this a lot at times however, and that's where the source of the problem is. I'm wondering how Sally might perhaps tip the scales in the end for Angua, if she thinks Carrot's going to be tempted in that direction