Where are the big amusement parks in the NW

Besides Silverwood, there seems to be a dearth of amusement parks in the I-5 corridor in the NW. Wild Waves isn't much bigger than a state fair midway and Oaks in PDX is ancient.

Any clue why the big companies: Great America, Six Flags has overlooked this area?
Our summers are warm and winters are nearly as brutal as New England, Chicago or Wisconsin.

I just watched a show on the travel channel about the all the parks in the US and thought of this question.

Thanks!