If you're insinuating that he thinks he couldn't have won, I think you're very wrong (both that he couldn't win, and even moreso that he would think he couldn't win). His region has been highly supportive of him for a long time and he's been very up front about his ideologies. Also, the campaign finance numbers from the beginning of 2020 before he decided not to run again looked far better for him even as an independent than it did for any of his Republican or Democratic competitors (in particular, he raised over 2x as much as Meijer in 4th quarter 2019 and also more than all his opponents combined, again as an independent at the time) [1].

The real reason he isn't running is because he spent 10 years in congress trying to make change and has decided it isn't the most effective thing he can do anymore. Congress is too focused on partisan politics and isn't interested at all in fixing its broken system (most of them seem to prefer the broken system), which leaves little room for anyone of any party to make real change unless they're the party leader (or in cahoots with them). I suspect we'll see him running for other positions in the future and wouldn't be surprised at all if he wins whatever race he decides to join next.

[1] https://www.mlive.com/news/grand-rapids/2020/02/independent-...