He didn't blow it up because he anticipated Batmans return, he wanted good for gotham, he wanted the Dark Knight to rise and defeat him, he never actually planned to blow Gotham up, that's why when the BatJet blew up it barely made an explosion, Bane actually stood for the good of all people and saw Gotham as a sesspit that had a hero in Batman (Bane knew that Bruce Wayne was Batman) that was hiding and letting Gothams scumbag big wigs run it and corrupt it, Bane knew that he had to smoke Batman out and when he did he knew that Batman would challenge him, but he also knew that Bruce Wayne would be the one to turn up dressed as Batman, so he beat him down but didn't kill him, he put him in the prison of the League Of Shadows to see if Bruce Wayne really could rise and become the Batman again to save Gotham, what he didn't take into account was that Bruce Wayne had had relations with Talia and wanted out of the crimefighting business. That's why Bane didn't blow Gotham up, he was merely a scare mongerer for the greater of good, he got the Dark Knight to Rise.