i think it's simply because in a zone, you have to be aware of more than just your job. read, recognize, react....and understand what your teammates' responsibilities are, and how it's all supposed to fit together...... that's a mental process that takes a little longer than "go get the guy with the ball" or "stick on this dude here".
as they do it more, get more reps in practice, those mental process speed up and they'll just be able to play.
perfect example was Crouch against MO, when he got beat by the TE. he read, he recognized, but by the time he recognized, it was too late, and he was a step behind the guy and out of position. it was a good throw too, to a tall target, but had Crouch been able to just hold his spot and take the TE right at the snap...who knows...maybe he baits the QB and gets a pick...or at least a pbu.
i don't think passion or want to have much to do with it....i think zone just requires a little more preparation. and really, the only way you get better at it is to do it, put stuff on film and learn from it.....that just takes a little longer to develop....i think by the end of the year, if everyone's still healthy, this defense will be pretty dang good, no matter what situation you put them in.