It's been explained so many times I'm getting bored just typing it out, but I'll give it one more try.
If there's a contested convention that denies the nomination to Trump, the frontrunner with the most votes, most delegates and most states won, the GOP will be split and Hillary will win.
So to oppose Trump at this late date is to help Hillary.
Think through the probable results of a floor fight where Cruz, by some miracle, wins the nomination. Do you really think he goes on to beat Hillary?
I have my doubts about Cruz's ability to beat her even if he'd won the majority of bound delegates prior to the convention, but it's too late for that. Now it's a bloody battle at the convention or nomination of the frontrunner.
Which would you prefer?
I would say that most of us Cruz supporters don’t think Trump can beat Hillary. Head to head polls show that Trump is by far the weakest of the three Republicans still in the race (he’s down 9.3 in the RCP average). I believe that a Trump nomination guarantees a Hillary victory.