Good question. His support in the GOP has held rock solid @ 30%-35% since this thing began. Nationally, his disapproval ratings are off the charts.. upwards to 60%.
The only answer I can offer is that this was all planned from the beginning. Trump is using the GOP to get a free platform for his screeds and insults knowing the media will give him free publicity. When the time comes, he will then bolt the GOP, take his 30-35% of his loyal GOP followers with him and start his own party. A 15-20% showing for the Trump Party in November will likely hand over the Presidency to Sanders or his friend, Hillary Rodham. Perot redux.
An then possibly Bernie feeling burned by the Hillary win with super delegates decides a 4 way race is anyone's ballgame and starts his own party.