'Whaddya guess is the real reason? '
He thinks the GOP is going to lose the House in the 2018 midterms.
There's nothing more that's going to be accomplished in this legislative session. If they lose the House, he's probably not going to maintain a leadership position. Even if he did, unlike Nancy Pelosi, he's got plenty of his political career ahead of him - so he's doesn't have a lot of incentive to just hang around the House waiting for his party to get the majority again. And there's nothing fun about being a backbencher when your party is in the minority.
So he's better off taking a few years in the private sector to earn his money, and wait for 2022 (Governor) or 2024 (Baldwin's Senate seat, or the GOP national ticket) if he wants to try to jump back into elected office.