The piece in the New York Times explains part of the appeal:
Mr. Trump’s strength among traditionally Democratic voters could pose some problems for his campaign. Many states bar voters registered with the other party from participating in partisan primaries. Other states go further, not allowing unaffiliated voters to vote in a primary; in the G.O.P. race, for example, that would mean restricting the electorate to those registered as Republicans — one of Mr. Trump’s weakest groups. This group of states includes many favorable to Mr. Trump, like Florida, Pennsylvania and New York.