Romney occupies a sweet spot in between where he is not far removed from the rigors of a national campaign, he is still a capable defender of his policy positions in the media and he is well known.
The ex-governor has the money to launch such a campaign even at the last minute. Thanks to his high name recognition, he would probably be able to win a lot more votes than some of the more obscure alternatives.
Finally, I suspect, though I obviously don’t know, that Romney regrets deferring to Jeb Bush and company at the beginning of the Republican primaries.
At the very least, it shows how wide a net anti-Trump conservatives are willing to cast to find an alternative to the likely Republican nominee.