A few reasons. One is there isn't much flat land; most of it is hilly and even mountainous and covered in thick forests. The flat areas are occupied with farms and towns but the space is small and not enough for big cities to grow. The hills and mountains are heavily forested and there has never been a big enough population to need to encroach on them. It's also not great for building and farming, unless grazing animals.
The other big reason is there are no natural deep sea ports in that region. It's either marshy or the estuary of the river Colombia. Small fishing towns would be fine, but not big industrial ports that drive city growth (or did in the past). Meanwhile, Portland sits further back up the river with plenty of flat land and access to the water, so makes a natural port. And Seattle sits on the bay further north and is coastal, and a good port.
The dynamic got set up of big cities further back, and those areas never really grew. Once the land became part of state forests, then that restricts growth even more.
