Seattle is a popular port for Alaska cruises. However, ships that sail round trip from Seattle (or any US port) have to make a port call at a foreign port, and most do so in Victoria. It's a short stop - usually from ~6 or 7 p.m. until 11:59 p.m. On
DCL - the port stop in Victoria had the ripple effect of having to depart from Ketchikan (and Juneau, I think) earlier. By departing from Vancouver, DCL spends more time in the ports, plus it has the more scenic sailing between Vancouver Island and the mainland.