Three reasons:
- His views on immigration have pulled the GOP to the far right on this issue, thereby alienating Hispanics, other minorities, and moderate white voters.
- Less obviously, his success as an entertainer has attracted millions of new viewers to the debates, which will make it more difficult for the ultimate GOP nominee to disavow statements made during the debates and pivot to the center during the general election.
- Instead of attempting to unify the party, he is doing everything he possibly can to publicly expose the fault lines between the GOP factions and the weaknesses of his opponents. The eventual nominee will consequently enter the general election with a divided party and with his shortcomings a matter of public record.