First, just because an adversary understands how a given steganography app works, or knows that it exists doesn't mean that they can detect the specific communication that's occurring, or will move to block that communication.
The canonical image hiding stego applications are a case in point, where the applications are widely distributed and understood, but in principle (if not in practice due to steganalysis[1]) one could know of their existence and how they work but still be unable to detect that covert communication through them was going on, nor be able to block that communication short of blocking all image posting.
Second, they need not be on any app store.
Third, any leaks about their existence, if they come at all, may come too late. As Napoleon said, it's not necessary to censor the news -- it's sufficient to delay it until it no longer matters.
