Because he didn’t want Trump to make the public aware of it because he knew it was the bogus reason for the entire investigation, and the entire game would be discovered.
Remember that Rosenstein is still withholding massive amounts of information from the House ain’t Committee despite subpoenas. Innocent men don’t hide things. None of what is being requested is a risk to national security, and based on the leaks from the IC, all of this info is already compromised.