The folks in Idaho have done a great job of marketing The Route of The Hiawatha Trail, and the tie - in with the Milwaukee Road has ingrained itself with the locals, as well as trail riders. I'm guessing ( based on my 4 visits there ) that the Hiawatha Trail also has many more riders than the Washington. In fact, Northern Idaho has quite a few rails to trails, and their State Tourism dept. makes sure we know about them.
I wish I could say the same about Washington's efforts towards both the John Wayne and the Iron Goat trails. A few years ago, there was a service offered that would haul you & your bike from Rattlesnake Lake Park in North Bend to Hyak, allowing you to ride downhill back to your parked vehicle at North Bend. The wife & I rode it on a sunny Sunday morning, with only one other passenger. This service died for lack of ridership, yet information was hard to find; many bike friends of mine had never heard of it. The similar service on Hiawatha Trail has long waits on nice days for the return bus ride from Pearson back up to Ronald, ID. Now, I'll say that St. Paul Pass is more spectacular than Snoqualmie Pass, but Snoqualmie is closer to a huge population.