This has definitely happened. Wikileaks refused to release info on Russia during the 2016 election. Wikileaks may have started out with good intentions, but it seems to be making some explicitly political actions.
There's plenty of legitimate reasons to not release information. Verifying the informations accuracy, that it hasn't been tampered with, and the source is credible is a really high bar to meet. Just because someone dumps data doesn't mean WikiLeaks is going to publish it