Warrior preps are almost all single use and many have to fit in your inventory as they are limited. Equally, their limited nature makes them harder to keep hold of.

Mage preps are multiple use, and plenty of them are effectively unlimited when it comes to replacing them.

So a warrior has to do more work to prep, and gets less out of it. For example, you hit a cabal and the warrior comes out protected, you back of, and hit it again shortly after. There's a very good chance he's missing some preps now. With a mage, he'll probably just zap/brandish again. And when he has to zoom off to get the prep, he can probably gather 3 or 4 uses of it, in the time the warrior gets one (if he's lucky).