Good answers - more fish, more wild fish, better genetics. Here's another possibility:
Proximity to the feeding grounds.

Many Pacific salmon stocks feed in the nutrient rich Gulf of Alaska. It's well know that the farther the salmon get from the feeding grounds, the less apt they are to feed. In Alaska, the feeding grounds are only a stone's throw away. In the PNW, the feeding grounds are a thousand miles away. By the time they hit the Hoh or the Kalama River (or anywhere near where I'm fishing), they've lost any instinct to feed. Thus they're tougher to catch. In Alaska, they may still have the urge to feed when they enter freshwater.

But this is only speculation. The other answers are probably more accurate.