Unless there would be some sort of major change in the electorate in the near future, which we have no reason to expect, Republicans can't win the White House this year without getting many millions of votes from non-conservatives. And a lot of the ones who are most persuadable are low-information voters, who decide how to vote based on factors as shallow as race, physical appearance, communication skills, whether a candidate came from a wealthy or more modest background, or what news items are most prominent just before the election (e.g., around fifteen percent of voters said that Hurricane Sandy was the biggest factor in determining which presidential candidate they voted for in 2012). Cruz might win over enough of those people. You could see him having some good moments in the debates or some good television ads, for example, that would make him likeable enough to those voters. Or his Democratic opponent could be flawed enough to allow Cruz to win in spite of his weaknesses. But we can't assume, without evidence, that some such scenario will occur. We have to make a judgment about Cruz's ability to win over the non-conservatives he needs based on what seems likely to happen, not just what might happen.
And that's a big problem for Cruz. He has an unusually bad reputation. (That's not just relevant to the presidential campaign, but also to how a Cruz candidacy would affect Congressional races and other campaigns. And it's relevant to how effectively Cruz would be able to govern if elected, for example. His bad reputation can't be explained solely on the basis of his conservatism, since so many other highly conservative members of Congress have a significantly better reputation than Cruz does.) He has a voice and facial expressions that are off-putting to a lot of people. The web and television are filled with shots of Cruz with a scowl or smirk on his face. The media and other opponents have an interest in portraying him that way, and they often do. But it's also a matter of what Cruz looks like and how he expresses himself. He often comes across as overly scripted, sometimes too dramatic or shrill. He's occasionally compared to a televangelist, and there's some merit to the comparison. He has a lot of strengths to counterbalance his weaknesses. He's intelligent, highly knowledgeable of the issues, comes from a modest background, is part of a racial minority, etc. But his weaknesses are substantial enough to make it an uphill battle for him to win over enough non-conservative voters.
And he has a competitor, Marco Rubio, who would be much more likely to get those voters to support him. He has a better reputation, looks and sounds better, and is a better communicator. He comes across as too scripted at times, but not as much as Cruz does. He doesn't motivate his opponents as much as Cruz motivates his. And he would increase the Republicans' chances of winning Florida, which is more important than increasing our chances of winning Texas.
There are a lot of irrational responses to Rubio's advantages, but one of the most popular is to claim that he isn't conservative enough. As I've said before, Rubio's recent ratings with the American Conservative Union and the Heritage Foundation are 98% and 94%, respectively. It's not as though he's a liberal or moderate. There isn't much of a difference between Rubio and Cruz in terms of conservatism. And you can't implement conservatism without first getting elected. Rubio's electability advantage, in an election Republicans need so much to win, is weightier than Cruz's policy advantages.