We're talking about Seattle because it is in the headline (the source is the Seattle Times). But there are 10 cities in the USA that have a higher percentage of "never-married single men." So this is a trend that is not unique to Seattle.
I just wanted to call that out since many of the comments seem focused on diagnosing what it is about Seattle specifically to lead to this trend.
IMO, it is mostly a housing issue. It seems like most new construction goes towards apartment complexes where the units are all 1-2 bedrooms, or else they go to more "luxury" big houses. I personally would love a 5 bedroom apartment in a dense, walkable city, but good luck finding it.
So I'm currently in the suburbs with my kids, even though I hate the suburbs.