Cowlitzfisherman
I don't really know the reason, am not very familar with that system. What is your guess?

For the Cowlitz, it could be that their habitat has been blocked by the dams and the main Cowlitz below the dams is lacking in their habitat needs - likely spawning gravels.

How many lampreys a year are you seeing on the Toutle/Green? A handful, 100s, or 1,000s? As far as that system goes it may have been that unaffected tributaries supplied a refuge from the devastation. As with the steelhead they may have been able to take advantage of the recovering habitats over the last 20 years.

My quick thoughts based on almost no first hand information.

Tight lines
Smalma