I was surprised too. Cardinals supporters used to turn out pretty heavily in RFK days and on into the first season or two at Nats Park - not as heavily as Mets or Cubs supporters at that time, but enough that if one was in the loo or on a beer queue and a roar went up from the crowd, one couldn't tell whether it was a good play by the Nats or the Cards.
I think a very large percentage of the crowd was STH and their additional purchases. By the time the general public got to buy, there were not many large blocks left, and without large blocks, the isolated singles and pairs were swallowed up. Cards fan bought out a few of the suites (I heard them).