It really just seems to be demographics, doesn’t it? Fewer families with young children, and a culture that emphasizes authenticity over cultivated themed experiences.
It's funny how enthusiast groups always say poor PNW but fail to realize how culture is sometimes different. That's not to say there aren't a lot of people there that would love a big park with a dozen coasters but it's all relative.
One of the biggest problems the area faces is an in-between park probably couldn't survive. They either need to be small to function or they need to be a massive destination park to draw people in. Just take KD for example..... The park has it's locals but it also sits on I-95 which can bring people from distances because they have something worth traveling for -- and you unfortunately can't build a KD sized park, let alone a Disney or Universal overnight.