Christie never stood a chance to win the GOP presidential nomination, especially as his campaign was focused entirely on opposing Donald Trump, so his departure is indeed quite logical and expected. What's surprising about his decision is that he believes that his few supporters will help another alternative to Trump, though not even he can name a better option for the White House.
Back in 2016, Christie made the terrible mistake of endorsing the then-outsider Donald Trump for the Republican nomination. Now, he and others within the GOP have realized that if Trump ever returns to the White House, he may establish an authoritarian regime. Nikki Haley seems to be the only viable alternative to prevent that outcome — and winning New Hampshire is crucial to that effort.