There's a lot of reasons. NFL owners are all participants in a massive NFL conglomerate, and the Bears are one of the flagship teams. The McCaskeys are rich...but all their money is tied up in the team. They don't have external wealth to rely on here, like most mega-rich owners. They just haven't been able to keep up with the Jerry Jones's.
Also, if the Bears fail to get the stadium done, because of failing to conquer political issues, that can have an effect on other teams as they attempt to do similar deals.