they aren't driving a car, they are driving several thousand pounds around and need to have their complete attention on where they are going and all the environment around them.
ANY distractions over and above the normal surrounding distractions are not a good thing.
Now there is no precedent set for this really it just never has been an option considered on most lifts. I have seen radio's installed in large yard lifts with cab's on them but for the most part a radio is just an additional cost and distraction that just isn't needed and not ever thought of as any sort of option on forklifts.
And the argument of the assembly department having a radio playing while drivers don't?
Well honestly the reason is pretty clear.
Plus, have you ever been in a place where there are multiple radio's playing different music? frankly THAT is MORE distracting.