The prevailing theory on that mystery is that Trump is doing to torture Romney. While Conservative news outlets certainly like that theory, it doesn't seem likely. After all, Trump walked the high road through his entire campaign when he often had the chance to get even, and there's nothing to gain by wasiting time interviewing Romney.
“Donald Trump was interviewing Mitt Romney for secretary of State in order to torture him,” Stone said on conspiracy theory website InfoWars.
“To toy with him. And given the history, that’s completely understandable. Mitt Romney crossed a line. He didn’t just oppose Trump, which is his democratic right, he called him a phony and a fraud. And a con man. And that’s not the kind of man you want as Secretary of State.”
There's still a lot of venom flowing from the media on both sides of the aisle, so at this point, the reason for Trump interviewing Romney is anybody's guess, but one does get the feeling that Trump was serving Romney a bit of crow, and helping solidify his reputation as someone you don't want to mess with.