Possibly because a brokered convention would result in Trump running as an independent. Yeah I know, he promised not to.
Trump talked about that at his March 5 press conference, it’s a must watch. It’s around 5:36 in the video.
Donald Trump Full Press Conference in West Palm Beach, FL (3-5-16)
https://www.youtube.com/watch?v=qWZHgZ5L004&feature=player_detailpage#t=336