Because new hatcheries are steel and concrete ( mostly ) and cost a lot to build. What was considered a good location for a facility 50 years ago is not necessarily a good location today. Hatcheries are sited around water quality and quantity, harvesters, and politicians desires. A good example is the Skookumchuck and Satsop Springs facilities in he Chehalis. Both were part of the Governor Evans state wide package to expand harvest opportunity. Once Gov Dan was out of office WDF shut the Springs down and went to raising Coho for the South Sound net pens at Skookumchuck.

You really have to know the history in any given area of the state to have any idea on how and why things are what they are now.