I can't even imagine the amount of money Vince gave him for this, it's probably not the type of offer Austin got before. I also buy into the idea that Austin saw it as a special opportunity to go full circle on his career (first and last match in the same place) and of he it wasn't tonight, he wouldn't have been able to do it like he he just did until it was too late. He could've done in WM 32 but that card had Rock and HHH trying to have a HHH moment with Roman in the main event, so much less room than in a 2-night event with practically no big names.
And supposedly he WAS interested in a match with Punk. WWE fucking up Punk's momentum and Rock/Cena having the main event locked up (and Punk dropping the ball badly on that sitdown interview they did together for WWE2K, I'll always believed that played a factor too) probably made him reconsider.