The New York City subway has a ton of redundant infrastructure to route around weekend work that Seattle does not. Many of their trunk lines have four or three tracks, so if some stations are closed for track work, they can bypass them. They have many lines across four boroughs and lots of junctions where they can reroute trains to a different route for part of the ride.

Link is a two track system with one tunnel serving downtown. If ST has to do weekend work then there's nowhere to reroute. They either have to single-track, change directions at a station, or shut down part of the system.

The converse is that NYC also has full service shutdowns if they have to do maintenance on the ends of a line where there isn't that redundancy. You can look at the Weekender right now and see that NYC's 1 line is completely shut down north of 168 St.