Because some of the vehicles will not be armored.
But we have no knowledge that any of the "some" unarmored vehicles would have been occupied.
If over half of the unit was flying in, obviously some of the vehicles were planned to be trucked in, not driven.
http://209.215.156.25/Pressrel/04Dec/04-61%20Statement%20on%20Rumsfeld%20question%2008%20Dec%2004.htm
Maj. Gen. Gus L. Hargett, Adjutant General of Tennessee
Statement on questions by 278th Soldier to Secretary Rumsfeld
...I have been assured that no 278th soldier will go forward in a vehicle without at least “level 3” armor.
According to my information, 278th Vehicles that are not armored will be trucked forward and will not be occupied by soldiers... "