Most Alaskan cruises stop in Vancouver, BC instead. That way it also capitalizes on Canadian cruisegoers, and it's a pretty good (albeit similar to Seattle) stop.
It's both a good hop-on point for Canadians, and also gets around the Jones act without seemingly random stops like Prince Rupert.
Realistically, if you're sailing out of Vancouver or Victoria, it makes little sense not to start a bit further south in Seattle. And if you're starting in Seattle and going to Hawaii or Alaska, you have to stop somewhere in between due to said act.