The Nats are in the 6th-highest-populated metropolitan area in the United States. The year they won the World Series they were 16th in average home attendance in MLB. They still don't have great attendance even when winning.
Vegas is the 29th-largest metro area in the US - 1/3 the size of D.C. They just got two new major pro sports franchises in the past six years and they're about to inherit a franchise in shambles that they don't even necessarily want. That franchise is gonna need all the help they can get.