I agree. But I came here from the Midwest. Detroit sports fans are in my opinion the most die hard despite lots of losing, so maybe I’m biased.
The Demographics of DC hurt it a lot in this respect. Lots of transplants (I’m one), so a lots of fans haven’t been fans since childhood. And a lot of people here came as fans of other teams. Plus DC is an event town. People like to be involved when it’s a big event. And a losing team isn’t really an event.
Interestingly enough, there are some cities/teams that still manage to make their games "events" despite losing. Namely LA teams (specifically the Dodgers and Lakers) and the Cubs (thanks to the vibe of Wrigley). The Nats don't have any of the pull factors (celebrities, a historic/epic venue) that those teams have, though, so there's no way in hell they'd ever get that kind of "event" status.
I do think "the ultimate bandwagon town" is a huge stretch. It's definitely a bandwagon town, but DC is absurdly loyal when it comes to football (the Redskins under Snyder hardly deserve fans, let alone a devout fanbase the size of which they still have) and more bandwagon for everything else. That describes a lot of markets in the U.S. I've lived in Texas for a few years now and heard nary a peep about baseball outside of the Astros' run. Even then, it was kind of tepid.
Florida is also notoriously fickle, especially Miami. I'd call Miami a much bigger bandwagon town than DC, in fact.
And then look at what happened with the Phillies these last few years. Attendance under 2 million in a larger market than DC.