Because Faith was so screwed up in so many different ways that I think she pretty much just completely snapped by the time she woke up from her coma in S4. Wilkins was gone, Buffy won the war in S3, Angel was still alive (and not with Buffy, which seemed to really piss Faith off more than it should have) and she pretty much had nothing going for her. That doesn't even touch on the avalanche of mental issues she'd picked up over the years.
She came back, thanks entirely to Angel, but at that point she was just lashing out at anything and everything around her, with a big emphasis on Buffy (who she was always jealous of, and, from the way I saw it, had a bit of a crush on). Joyce was just in the way.