Here's a couple more reasons to add to those already mentioned:

In the ol' days, you could fish waters that aren't even open, today. The Ballard Ship Canal was a popular spot for a lot of Seattle steelheaders. The Cedar River, King County's third largest stream, was very popular with south end anglers. It has been closed to steelheading for almost a decade. The Sammamish Slough was accessible along most of its length, and was heavily fished by eastside residents. It has been off-limits to steelheaders for almost as long as the Cedar. A favorite spot of mine was Tokul Creek, above the hatchery and up into the canyon. That area was closed to fishing when the gun range was built. Issaquah Creek no longer offers winter fishing opportunity. When you add to this list the number of fishing spots that have been privately developed and posted, you can see why many of the remaining accessible areas have become crowded.

Seattle area steelheading was much better in the 50's, 60's, and 70's, than it is today. You didn't have to go to the OP, or to SW Washington, to find good fishing. Now the rivers of those regions are becoming crowded as many anglers are more than willing to make the commute to find fishing as good as it was in the past.