Cruz didn't spend the past 10 years or so crafting an apolitical pop culture brand image as a brilliant billionaire businessman genius behind big impressive skyscrapers and casinos via self-promoting media campaigns and a seven-year prime time reality TV show.
That might have something to do with it. Palin had rallies of 10K strong, but few heard about it because the MSM failed to report it -- understandable, as Palin didn't create her own brand image through the MSM the way Trump did, Palin created it out of her political stances and political actions.
The MSM ignored and then attacked Palin -- and that same MSM breathlessly awaits Trump's every controversial word because he is a media-created celebrity very familiar to them.
IOW, Cruz doesn't have large crowds because he's not a pop-culture icon. The truth is, Cruz is running the Huckabee/Santorum/Michelle Bachman campaign. Without Trump in the race, his best bet would be Iowa, then he would have lost to Jeb or Rubio somewhere along the way to Florida, wherein Cruz would be kookified by quoting his daddy talk about the "End Times Transfer of Wealth to the righteous" or showing his weaknesses on TPP and immigration, which render his opposition to Rubio et al entirely pointless.
Trump's campaign rules because it has all the energy in the race, bringing in new voters left and right. Once the actual votes begin, Trump's already sizable lead will probably come out far far larger than even the best polls suggest.
Cruz cannot create that buzz. Too artificial, too vulnerable.