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

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)


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

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.