It's not odd, that was the strategy. The Snohomish County Council did not want PAE to be a large commercial airport and built it this way (small gate capacity and not a lot of infrastructure) because there was a lot of local backlash at the idea of having a commercial airport operation in that location, but there was an FAA requirement to allow commercial service (so it was deliberately capped pretty small). You can dig up the old FT threads if you'd like. Very similar to how LGB gets kneecapped by the locals.

Candidly this should be rethought because if the choice is "build a hyper-expensive new airport, or better use the ones we have already built at PAE and BFI to supplement and relieve SEA" the answer is fairly obvious.