Because admitting that Snowden was a whistleblower or a patriot would imply that they had not been doing their jobs as overseers. It would also potentially limit the power of the intelligence apparatus, which would in turn limit the power of these congressmen as some of their clout comes from being on a committee overseeing a constellation of powerful agencies. Their incentives are completely misaligned.

That depends on the members of the committee. Are they dedicated to substantive oversight, or a rubber-stamp process? This controversy ultimately boils down to a lack of oversight and accountability. The current spying regime maintains oversight by pliant secret courts and congressional committees is enough. The scale of the spying powers has not been met by equally powerful oversight.