This seems hastily arranged. My bet is they were holding this endorsement until after Tuesday but felt the need to pull the pin after last night's debate. Just speculation but campaigns often sit on endorsements waiting to time them for max impact
Why would you wait for arguably the biggest endorsement you could hope for until after the biggest primary day?
Because Christie's endorsement does Trump little good (and maybe some harm) in TX, OK, AR and other Southern states, while reinforcing Cruz's narrative about Trump: that he's neither conservative nor even really Republican.