I see several reasons why, and more than one can be true at once:
1. May not want to do so in case other people he knows are indicted for stripping classification notices off messages before sending them, knowingly sending confidential info to groups for political gain and providing info in exchange for bribes. While I doubt he's involved, some of his peers could be, so he's silent for their sake.
2. He may have been advised NOT to comment on a pending investigation by legal advisers, especially if he's President when charges finally go down or trials are ongoing.
3. He thinks he has plenty of other topics that are less complicated to bash her on.
4. He doesn't think it was bad.
5. Let Trump hammer her for it. It pulls Hillary down to allow Sanders to win, while he doesn't have to enter the fray. That makes him look like the cleaner candidate.