Because this is a case of alleged collusion vs alleged coup neither side can claim the right to keep a secret.
Ordinarily one party represents the state in an investigation. But the question in this case is precisely who is the state? Because this is ultimately a political struggle there are precious few rules either side can hide behind. The Marquis of Queensbury has left the building. They have bet the farm, both of them.
https://www.chicagotribune.com/columns/john-kass/ct-donald-trump-whistleblower-kass-20191109-4y5nwaa4enbvrbpfq7gfpwdrba-story.html