Why do the other candidates, especially Cruz and Rubio, think they can't win over Trump voters?

Trump in the first three contests has received on average a third of the primary vote netting him two firsts and a second. No other candidate has yet to break 30% and it seems like with the divided field Trump is poised to romp through the March primaries except possibly in small caucuses and the home states where the other candidates hale from (FL, OH, and TX).

From polling, Trump's core support appears to be lower-to-middle income, non-college educated voters who are not especially religious nor conservative, though Trump gets his fair share of that vote as well. Cruz seems to be targeting highly religious evangelicals and very conservative voters who want a purist candidate above all else. Rubio is aiming for college-educated, upscale suburban/ex-urban voters with his optimistic appeal to a new face [his] of conservatism.

However, neither Cruz nor Rubio has attempted, except in brief passing, to go hard at Trump's strength which is his promise to his supporters to "Make America Great Again." Where are their ideas and policies that soothe the anger and anxieties of these blue collar voters that make them turn to Trump, who even they acknowledge is bombastic, crude, shallow, and not really a conservative?

If either Cruz or Rubio could give them concrete answers to rebut Trump's demagoguery, I'm sure a significant fraction would peel off to their candidacies and reveal that the rock- solid support for Trump is really fairly brittle. Until they do it convincingly however, Trump will retain his third of the vote and likely be the GOP nominee.