Six Flags is not the only one hurting. Across the board and around the country many amusement parks are charging entrance fees that are out of this world high. Not only do pay upwards of $100 sometimes just to walk to door, but then if you or anyone you're with wants to eat, look out! Souvenirs, way too high; a memorial t-shirt of your trip is about 3 times as expensive as if you were to buy it offline or at a shop in town.
The state of economy in general is what determines how much extra people have to spend at places like SIx Flags Parks. When money is tight entertainment spending is among the first things that families choose spending money on. While the unemployment rate has dropped there are still a large number of people who are still working part-time hours.
At the end of the day, Six Flags is a place where people go for leisure, and as the economy continues to slide, people have less money for leisure. Where a family may have visited two or three times a year, they might now only go once. Other families may not be able to go at all. This is simply how things work when times become tougher.
The economy has actually been better in the last few years, but the weather sure has been worse. Because of how long of a winter many states have had, I think that most people are outside having fun and once their kids start to become bored closer to fall, they'll start going to the amusement park again.