The reason is because the plateau was formed about 5700 years ago by a volcanic mudflow (lahar) from Mt Rainier.

A huge landslide removed 3 cubic kilometers (0.7 cubic miles) from the summit of Mount Rainier. (The landslide removed the top 600 meters (2,000 feet), leaving a summit crater. Subsequent volcanic eruptions created the modern summit cone with the crater.). This one single event created 200 square miles of land in a matter of hours, with waves of mud 20 feet to 600 feet high. This wall of mud had the consistency of wet concrete and traveled up to 60 mph. This mudflow destroyed everything in its path, uprooting entire old-growth forests. It hit Puget Sound with such force and with so much material that it flowed underwater for 15 miles, maybe farther. An area of hundreds of square miles was covered with mud and debris up to 350 feet deep.