The most likely answer, theorize Republican strategists watching this play out, is that Chaffetz is just bored with his job. His job as chairman of the oversight panel is to investigate the government, and it probably would have been a lot more fun for this tea party-leaning Republican to investigate Hillary Clinton's government than President Trump's.
After Trump was inaugurated, Chaffetz rather provocatively suggested that he planned to remain a national figure because of Clinton, not Trump. That hasn't really materialized now that Congress is investigating Trump's potential ties to Russia.
The House and Senate intelligence committees are taking the lead on that, but Chaffetz has ducked calls to have his committee begin an investigation (earning much criticism from the left). He also has ducked some of Trump's own investigation requests. When Trump demanded that Congress investigate voter fraud, Chaffetz shrugged: “The president has 100,000 people at the Department of Justice, and if he wants to have an investigation, have at it,” he told CNN.
So what does that leave Chaffetz to do? Not much. And in a town full of overly ambitious people, he is especially known for dripping with ambition. It's possible that he got bored in his job and saw an opportunity elsewhere.
He certainly hinted at that in his Facebook retirement announcement: “After more than 1,500 nights away from my home, it is time. I may run again for public office, but not in 2018.”
Where elsewhere? Well two-term Gov. Gary R. Herbert (R) announced that he wouldn't seek a third term in 2020. Josh Romney, Mitt Romney's son, said he's “strongly considering” running, but that wouldn't preclude Chaffetz from jumping in. “Definitely, maybe,” Chaffetz cheekily told the Atlantic's McKay Coppins just a few weeks ago when asked whether he'd run.
Chaffetz could certainly run for the governor's mansion from Congress, but maybe he just decided that his name recognition was high enough to do it from the private sector (where in-the-know members of Congress can make millions) than a so-so job. Also, he was bored.
Unlike Health and Human Services Secretary Tom Price's Atlanta-area seat, Chaffetz's district isn't going to suddenly be competitive for Democrats. It's one of the most Republican districts in the nation.
But he was under a lot of pressure from the left lately, which, in the context of everything else, may also not have been fun for him. In February, Chaffetz was ground zero for the first round of raucous Trump-era town hall meetings. A few weeks later, he quickly acknowledged that he wasn't “smooth” when he said this on CNN during the health-care debate, even though it's something that is in line with years of Republican orthodoxy.
“Americans have choices, and they've got to make a choice. And so maybe rather than getting that new iPhone that they just love and they want to go spend hundreds of dollars on that, maybe they should invest in their own health care.”
Chaffetz has never been a traditional politician. He makes political calculations slightly differently than the average lawmaker, and he doesn't really care if he raises eyebrows doing so.
About two weeks later, he walked that back with this:
Trump was still not expected to win the election. But Chaffetz, we theorized, was gambling that his political future depended on at least supporting Trump/opposing Clinton. When John A. Boehner (R-Ohio) resigned last year as House speaker, Chaffetz announced a run for speaker. (He dropped out after Rep. Paul D. Ryan entered the race.) Supporting your party's nominee for president was kind of a minimal requirement for being speaker.
But Chaffetz has clearly decided that speaker is no longer a job he wants. (Watching Ryan take heat from his party for the recent health-care debacle probably didn't help sell the job.)
Chaffetz has passed up other political ladders to climb. He thought about, then decided against, challenging Sen. Orrin G. Hatch (R-Utah) in 2012. At the time, he said his upward mobility in the House was a factor.
This might be one of those cases in which the simplest answer is the right answer. And the simplest answer is that Chaffetz just decided he wanted out.