I’ll venture a guess or an opinion ... but why these hearings are being held behind closed doors is very telling of there is more wrong doing, going on, than We the Serfs are entitled to hear about. This is an opinion but has possibilities of being true based on what is currently known. Not everything is classified is another opinion.
A public hearing gives witnesses the ability and excuse to not answer due to information being classified. The House Armed Services Committee has cleverly taken that option off the table with a classified members only briefing.