He says it is because he thinks it is time for a new generation of leaders .
Mitt Romney won't seek second Senate term but isn't 'retiring from the fight' : NPR (https://www.npr.org/2023/09/13/1199272961/mitt-romney-reelection-2024)

I would applaud him for that view if it were real.

But I suspect that it has more to do with potential candidates building up a huge war chest to primary him .
Potential Romney primary challenger builds up $2.1 million war chest (axios.com) (https://www.axios.com/2023/07/11/mitt-romney-senate-utah-brad-wilson-fundraising)


That and the fact that the Repubs in his home state don't want him .

More than 60 Utah Republicans endorse primary challenger to Mitt Romney | Fox News (https://www.foxnews.com/politics/utah-republicans-primary-challenge-mitt-romney)

He'd rather bow out gracefully than spend his huge war chest fending off a challenge. Not to worry . He has a lucrative career ahead as a guest commentator on various compliant press talking head shows as long as he remains a "moderate" who attacks Repubs.