I wouldn't say no to the idea, but KF is based on 6-man, claustrophobic-type combat. Hallways, corridors, at best, streets filled with broken vehicles (I'm talking about the official maps, of course). The only map vehicle-friendly is KF-Farm, and even that has some big hills that could cause trouble for transportation.
To make vehicles for this, it would mean that:
a) You'd have to make a big-*** map(s) with clean terrain (streets, fields etc.) to drive on;
b) You would have to make the vehicles not overpowered (a tank against a bunch of zombies so slow my grandmother could outrun then? Hell, even the Flesh Pound wouldn't be so scary anymore);
c) You'd have to make a way for vehicles to be 6-man friendly. APCs that give the 4 soldiers carried inside a use, not just the driver and gunner. Tanks that can carry a number of soldiers on it and/or are extremely slow so that soldiers on foot can keep up. Armored Vehicles (most likely HMMVEs, since everyone get's a stiffy from those) that can carry 6 soldiers without giving attributes to 2 of them while making the others completely useless (same issue as for the APC).
d) You'd have to give them balanced weakpoints. Besides the Patriarch, there are no zombies that can fire from a large distance (Siren doesn't count, she can hurt only those in an area around her and she's slow as hell).
It's way to much work for the game team who right now are probably trying to balance out current content and make new, easy-to-make, fun for the original experience, content. Vehicles would mean, pretty much, a whole new game.
And seriosuly, to just "port" vehicles from UT2004? Where would you drive them?