It isn’t. Iowa went to Huckabee in 2008 and Santorum in 2012. It means nothing.
However, a win is a win, and just the fact that Cruz won the state gives him headlines,visibility and perceived strength. The fact that the others lost shows vulnerability.
It went to Ford in 1976, Dole in 1996 and Bush in 2000, all of whom went on to win the GOP nomination. It's not everything, but it's not nothing. Leaving out GOP incumbent re-election campaigns, during which they ran unopposed, that's 3 out of 7 Iowa caucuses going to the eventual GOP nominee. Santorum's was a shoestring campaign - raising $22m by the end of the primary cycle. Cruz is at the start of the cycle, and had already raised $47m as of year-end 2015, before his Iowa victory.
Cruz has the firepower to go a few rounds with the Donald, unlike Santorum, who was outgunned by Romney's negative ads. And of course, Cruz hasn't even begun to go negative - he has mainly gone after Trump's past positions, temperament and ignorance of the issues that candidates show know about, instead of Trump's checkered biography. Whereas Trump has gone all-in on Cruz's biography what with the attacks on his eligibility and his loans from Goldman Sachs and Citibank.
If Trump thinks he's getting heat, he ain't seen nothing yet. Just wait till the race narrows down to Cruz or Rubio and Trump. Then he'll get the moral equivalent of napalm via negative ads that target his sleazy business deals - ads that will stick with potential business partners long after the 2016 campaign is over.