Yes, because they could tell it was really over for them and their money had dried up. Which hasn't happened this time! Cruz is much more annoying to Trump now than Santorum, who had more than a month still left in his run, was to Romney four years ago, because Rick had no money or other resources and thus had no chance. When the field narrowed for Mitt his percent of the remaining vote climbed. As Trump's field narrowed his percent has stayed fairly flat. Cruz has resources, a game plan and a good chance to beat Trump at the convention. Trump still has a chance because too many others stayed in too long, and in many cases gobbled votes from Cruz via early voting in races finished after their exits. Had Rubio and Carson gotten out, when they'd clearly lost, the race would be tied already.
Notice your double standard. "Waaaaaahmbulance, it's not FAIR, the other people kept Cruz from his rightful votes!"
Yeah, Trump faced the same crowded field that Cruz did. And beat all of them at the same time.
Cruz won his home state (< 50%) and OK (next to it); he won Utah (Mormon, and Romney friendly therefore GOP-e friendly); and Wisconsin (GOP-e home of Ryan and Priebus...and Walker endorsed Cruz)...and Cruz *still* didn't get 50%.
Plus, Trump has been facing the unified fire of Fox News, the MSM, and hundreds of millions in ads against him from GOP-e influenced PACs, as well as Limbaugh and Levin. Anyone else would have been reduced to smoldering ashes by now.
Incidentally, wasn't it Cruz who won Iowa by having surrogates (like, you know, Amanda "matching tattoos" Carpenter) suggest that Carson had dropped out?
And Trump managed to kick the living sh*t out of Rubio in Rubio's *home state* where by definition Rubio should have been strongest: and Trump took a plurality of the Evangelical voters in the Deep South, which is the only geographic region friendly to Cruz.
Cruz lost Florida and Ohio; he looks like losing Pennsylvania. Were he to win the nomination, he would lose in a landslide just by those three states: and that does not take into account his underhanded tactics with the GOP-e, which may drive off as many as 1/3 of Trump supporters.