Another thing that dampened attendance is that SPH exchanges were blacked out for this game. I know that you're basically drawing attendance away from one game to give it to the other when you allow exchanges, but nobody expects a packed house for a noon game on a Wednesday in July against the Brewers, so if you "only" get 20k people, who cares?
I have houseguests and a season plan with just 2 seats, and I've given the Nats tons of ticket money this year, so when they decided to black this series out, well, that is why I took my houseguests to two games against the Reds and none against the Cubs.