The people they are interviewing are not saying what they want to hear, far from it, and they don't want that to get out.
Republicans on the oversight committees were part of the meeting invaded by their unauthorized colleagues.
The closed nature of the meeting was because sensitive intelligence information pertaining to Ukraine operations was being reviewed by the committees. Ukraine, you may recall, is a focus of allegations the committees are investigating.
Intelligence methods and operations are sensitive and not to be made public for obvious reasons. Those on the oversight committees have been vetted for security and have been approved for access to this level of sensitive intelligence information. Others have not been vetted.
This wasn't some super secret Democrat conspiracy meeting on impeachment. It was a bipartisan information gathering meeting.
The invasion of the meeting by unauthorized House Republicans was a publicity stunt that embarrassed the nation, the House and the authorized Republicans attending the meeting. But it played to the Trump faithful, like yourself, who don't care about the details only the appearance.
Edited by AZPaul3, : No reason given.
Eschew obfuscation. Habituate elucidation.