It happened because a former mayor (Ed Murray, who has since left office after a personal scandal) made it easier for ISPs to build out their networks and removed homeowners and neighborhood groups from the process which previously allowed them to block things like communications boxes on sidewalks.
At the time, a bunch of local columnists moaned about this "corporate giveaway"[1] -- but it's pretty clear in retrospect it was the right move.
1. https://www.seattletimes.com/business/mayors-centurylink-giv...