Because it would give validity to the claims of these boards and force the discussion into the public. It's a game that is being played. POTUS can't be the one to say "we have the goods on these guys." It must get to such a point where the media has no choice but to try and rally the public against POTUS in defense of accused traitors and child molesters.
That is how the game is being played, a sort of game of peaknuckles where the media thinks it can bloody POTUS' knuckles before theirs are reduced to powder and raw nerve. They'll start screaming soon enough. But the beatings will not stop until the corruption is removed.