I think the reason is clear, particularly after Christi says he would consider being vice president. So here it is...
Christi wants to be president, but not yet. He said as much. Maybe next time. He also figures Romney will be the nominee. By endorsing Romney, maybe Romney will make him VP. That way, when Romney's time is up, Christi won't even have to fight hard or raise a lot of cash in order to become the next Republican nomineee - a VP is almost a shoo-in.
And there you have it. Christi endorsed Romney because Christi want to be president the next time around, and wants to avoid the usual hard-ball campaigning to become the nominee.
Except for one little thing - Romney may not get the Republican nomination. In which case the VP might well be Rubio or Jeb Bush, who would then go on to beat out Christi next time around because THAT person would be the shoo-in.