Amash is retiring at the end of his term; he would have had to run as a Libertarian against a Republican whose name is on half the grocery stores in the state (Meijer).
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-...
I do have to contest the first part of your statement - it's wishful thinking to see him having won this race. You're correct that he enjoys a broad base of bipartisan support, but the combination of turning on Trump, leaving the Republican party, and having a well-recognized opponent imperiled his chances in that district. He would have had to snag approximately 25% of the Democratic vote and 45% of the Republican vote to achieve that feat.