Two facts ... (1) Making south bridge chips is pretty hard, I assume ATi just didn't get theirs finished, and are nowhere near achieving that goal - so they took an existing one. (2) VIA's south bridges actually aren't bad (and the SB!Live desaster wasn't a south bridge problem at all, btw. It was a PCI signal quality issue _caused_ by the SB!Live, upsetting the VIA _north_ bridge).
regards, Peter