All four of them can't fix the elevator because when Leonard first met Sheldon, Sheldon saved Leonard's life. Leonard was about to go into an elevator with a pot of chemicals which was about to burst. Sheldon pushed Leonard out of the way, put the pot in the elevator, and closed the elevator doors. While Leonard is saying that he had plenty of time, あなた hear a huge crash in the elevator. And it is so badly damaged that no one could possibly fix it and if they did, they would have done it a long time ago
Well I guess It is not possible because when the elevator explode the キャビン got damaged. Although they may have the technical knowhow they could not repair the elevator because they do not have the materials. But I think the elevator will never be repaired because it is one of the most important running gags of the series!