Could be because they concentrate more on their squats than they do on speed- But the truth is, you can actually train for both and get good results- I've seen some pretty large men with monster legs who can really move. It's two different types of fitness, really- And sure, those who concentrate solely on strength will gain more strength than those who try to get both strength and speed- but doing squats and deadlifts is not necessarily a guarantee that you'll end up slow. It all depends on the person, and the type of training they're willing to do.
skill players leave squats to the linemen, let them get slower. : )
In actuality, this should read- Skill players add squats to their routine, and work even harder to keep up their speed. It makes a better athlete, in any sport, to have a good leg and lower back workout, bottom line.