I should note that it is Republican Senator Charles Grassley who is appearing to take the lead on this.

They want to see if too much of the tax exempt^* money is going to fund a lavish lifestyle.

Ole Anthony, of the Trinity Foundation has been trying to get coverage of this for years, and finally went to Grassley because the press would not cover it.

My guess is that part of the reason for the lack of press coverage is a fear of being attacked by the religious institutions, and the other reason is because churches are allowed to conceal their financial dealings in ways that other non-profits are not,^‡ which makes it tougher for the average reporter to get the information they need to make a story.

With the threat of congressional subpoenas, the information that Grassley gets should be more complete.

^*I actually have some experience with laws involving tax-exempt religious and educational institutions, section 501(c)3 of the tax code. In 1990, I incorporated a group that put on a literary conferences, Arisia as a Massachusetts non-profit and a federal 501(c)3^†.
^†Truth be told, in a sane world, it would have had to incorporate as a 501(c)7, a membership organization, but the 501(c)3 code is lax enough to slip it in, and we needed the low postage rates available for a 501(c)3, but not a 501(c)7. It was legal, as to the ethics, I try to think about Arisia as little as possible for any number of reasons.
^‡BTW, one of the problems about getting information about how churches are run is that unlike other tax exempt organizations, churches are not required to file form 990s, which are public documents with fairly full financial disclosure.