Like every Plumber in Washington, you were trained and tested using the UPC when you turned out as a Journeyman Plumber. You know the UPC, rely on it and chances are, you already own a copy. You also have a say in what goes into the UPC.
Adding another code in Washington would mean you would have to retrain using the IPC. Every county and city you work in throughout the day would have the option of requiring the UPC, the IPC, or in some cases, both. Each new job would come with a different set of rules for you to follow and two sets of code books you would have to purchase and follow.