They are so worried about the games ending during rush hour that they don't bother with it.
That could quite possibly be the reason - they are rather hypersensitive on traffic/access matters - I remember in the run-up to the opening of Nationals Park the front office was in such a panic over parking 'oh dear, we're only going to have parking for season ticket holders' but that turned out not to be the case - nowadays their ads on the jumbotron for tickets to upcoming series regularly feature 'single game parking available'.
...or are they? The mess we had getting out of the Fourth of July match last season on the Metro, fighting all those hordes of people going to the mall festivities, made me hope the Nats never played at home on the Fourth again - unfortunately, they will be home on the upcoming Fourth. against the Mets no less, so on the issue of parking/traffic/transportation it would appear that on this, as with so much else, the front office is giving itself proctoscopies with their heads.