Trump is so friggin' clueless and incompetent. He was SURE he would have the nomination wrapped up by now.
Or, as is more likely, he had no idea that securing delegates meant that he actually had to field slates of delegates friendly to him and woo them. And he had nobody in his campaign that had any familiarity with the process.
So, he will get his first ballot votes. After that, over half of them are likely to go to Cruz, Cruz will be the nominee, and Trump will pitch a fit.
He will deserve it, though. Hubris does that most of the time.