Because his reversal seems opportunistic.
Cruz was always loathed by the Republican leadership, and by some more establishment conservatives, for being an opportunist. This is a guy who helped promote the 2013 government shutdown for his own personal brand building, even though it hurt the party as a whole.
Throughout the 2016, presidential campaign, Cruz's attitude towards Trump has been marked by brazen opportunism. Early on, through at least the first couple of contests, he refused to criticize Trump. Even while other candidates were attacking Trump, Cruz refused, saying that Trump's presence in the race was great. Everyone saw what Cruz's calculation was: he thought that Trump would eventually flame out and he wanted to be positioned to inherit the Donald's supporters. It was only when it became clear that Trump was going to be in the race for the long haul that Cruz began criticizing him.
Then, at the RNC, Cruz made a speech where he refused to endorse Trump, instead telling people to "vote their conscience". At a time when the party was trying to come together behind the nominee, Cruz's comment was booed. Trump critics liked the speech, however, viewing it as a principled stand against a terrible candidate, and one who had treated Cruz terribly in the primary. They particularly contrasted it with Chris Christie's sycophantic reversal on Trump, calling him terrible before the New Hampshire primary and then becoming his biggest booster afterwards.
But Cruz's convention speech was also recognized as being self serving. Cruz was clearly laying the groundwork for a 2020 presidential campaign. He was making a bet. He was betting that Donald Trump would flame out in the general election and that he would lose badly. If that happened, as it seemed it might back then, the people who had opposed Trump energetically, as Cruz did at the convention, would be regarded as principled conservatives who had warned their party about nominating such a terrible candidate. When 2020 rolled around, Cruz would be well positioned to capture the nomination, being able to portray himself as one of the few national conservatives who had stood by their principles and opposed Trump.
But then things changed. In the last couple of weeks, Trump has pulled neck and neck with Clinton. He's won over 90% of Republicans to support him. There now looks like there's a very good chance that Trump could win. Cruz is also facing a potential primary challenge in 2018. A lot of Republicans were angered by his refusal to get behind the nominee and someone just announced that they intended to run for the Republican nomination for Cruz's Senate seat in 2018. So Cruz is in a bad place. If Trump wins in November then his refusal to endorse him won't look brave and principled. It will look traitorous. And he might not even be in the Senate come 2020, regardless of what happens this fall. Cruz needed to assuage Trump supporters in Texas, who might be inclined to vote against him. So he switched his previous position and agreed that he would support Trump.
The problem is that this is yet another nakedly self serving move by Cruz. Nothing about Trump has changed. He's still the same deranged liar who Ted Cruz was criticizing at the end of the primary. The only thing that's changed is the political calculus. So Ted Cruz's move to endorse Trump is seen for what it is: a blatant move by Cruz to try and save his political future. His endorsement is so self serving and comes so late in the game that I doubt he will get much credit for it. But he's also probably lost the support of the minority of conservatives who actually liked the stand he took against Trump. If you're an anti--Trump conservative then Cruz just turned into another coward who is willing to sell out the Republican party and conservative ideals.
· 1 year ago