Listening to Ferrell last night on 106.7 and he had a long segment on the decline of people going to baseball games. Cost of course was one of the factors. I'm trying to maintain my 10 to 15 games a year but cost is cutting in on me as well. Even on the cheap, it costs me more than $70 (2 Vienna Lagers) and 6 hours. But man, I love bein' at the Park.
There's also the flip side of the "build it downtown" push. Those who are nearby or have easy access to mass transit may well find other things to do at a better timing and cost control point. Mitigating factors like the Metro shutdown south of National Airport also play into it.
Those who want to get from point a to point b via car have the elevated parking cost and ingress/egress logistical challenges (both to and from parking and from parking to the ballpark) that such parks often possess and which impact timing. Particularly at Nats Park with the increased surrounding density, the ease or lack of it for the drivers plays into it.
A recent case in point was when my brother's in-laws were coming to town and were going to take his three young children to a day game (and bought a parking permit for Garage C). I was going to the game and offered to drive, which ended up with us sitting in backups a couple of blocks from the stadium and missing the start of the game despite leaving decently early. Street after street was backed up or blocked off, not helped by the fact that they had apparently turned Half St SE from M St. SE to the ballpark into a charter bus parking lot. His father-in-law practically wrote off going to more games there.
A longtime fan like me is just going to find an alternate approach (though I'm not exactly feeling great urgency to do so at present), but the more causal fan has a lower tipping point to reach, and I think many factors are in place for the tipping to be more dramatic going forward. It's a moot point now, but as the DOT arrival and Navy Yard infill was likely to lead to build up of that area eventually anyway (a decade after the ballpark opened), the insistence on that particular locale over the RFK Stadium area always struck me as less than ideal long term. That's especially the case for day-of-game ticket buyers both for driving and for Metro.