Romney appeals to the sort of people who own multiple houses. Most Trump supporters are working class concerned about the price of groceries, who would probably be better served by a left-wing pro-labor politician but won't vote for one because they abhor the progressive social policies that get bundled with that in America. So they vote for the guy who plays lip service to their problems while talking smack about progressive social stuff.

Romney is politically obsolete. He serves no further purpose in American politics.