Revealing their successes would probably just lead skeptics to believe they were engaging in propaganda, and it probably wouldn't act as a deterrent to terrorists at all.
Maybe... On the other hand, keeping silent doesn't seem to convert a whole lot of skeptics either. And revealing a couple of successes would probably result in a lot more votes for your administration the next elections, right?
It certainly could bring in political capital, but aren't politicians who do that already accused of manufacturing terrorist plots and ratcheting up fear for points during election season? How valuable is public relations for the US intelligence community versus keeping their methods secret (within a culture of secrecy?) It's not as though the CIA or NSA are likely to be defunded soon, regardless of who gets elected.