Chaffetz doesn't want to be in Congress when people expect productive things out of him.
It's easy to be an opposition-party representative, especially when you chair the House Committee on Oversight and Government Reform. All you do is vote "no", propose one-sentence feel-good bills that have zero chance of passage, and call for investigations of every little ninny thing the president does that you don't like. In short, you get to be a negative nellie and throw tantrums and hint and conspiracies and your base loves you.
Once Trump won, Chaffetz's role in DC suddenly (and very unexpectedly) shifted. Now he's expected to help put forward reasoned legislation that will actually become law, help advance the president's and party's agendas, and use his chairmanship to stifle and slow down the democrats from trying to do to Trump what Chaffetz did for Obama.
I think Chaffetz is extremely self-interested and has no interest whatsover in constructive governance. Once Hillary lost, his gravy-train to free media and self-promotion dried up. No one in the media cares about Hillary's emails, Fast and Furious, Benghazi, or any of that any more, so he's lost the lust for those inquiries, and he has no interest in taking hits for the team under Trump because there's no glory in stone-walling investigations into Trump's bumblings.
Chaffetz never thought Trump would win. If Hillary won, he'd be all Orrin Hatch on us -- telling us how important his seniority and position is and if people wanted term limits they'd vote him out. Instead, Trump ruined his party, so he's trying to leave with his "reputation" intact as a springboard into something else to bide the time until he makes a run at either the governor's seat or Hatch's seat in 2018.