There was a fourth GOP debate tonight, on a cable network with few viewers, moderated by Megyn Kelly who, it should be noted, had her warpaint on. Articles like this one in Politico, written after the debate, beat up on DeSantis, Haley, and Ramaswamy for not criticizing Trump, which of course Christie did.
Let's unpack this and determine if it makes sense. The only way any of the three will be the nominee is if - one way or another - something takes Trump out of the race. It could be some part of the lawfare he is experiencing, an unforeseen health issue, or some black swan event.
If that happens, whichever of these - likely DeSantis or Haley - is ahead of the others will be the convention's default choice. Nobody can admit it, but that is why three of them are there.
I'm not certain what Christie hopes to gain by being the anti-Trump. No Republican with that label will be elected president in the near future. An able guy, he hopes to profit in some other as-yet-undisclosed way.
The three who have a shot at being Trump's replacement, need his supporters to vote for them in the general election. A "default nominee" needs Trump stalwarts to believe that Trump losing out wasn't the nominee's fault. Hence, no criticism.