I think zone blocking works today because running backs are taught to run more like that in college and high school. It's more of the runner's decision as to what hole to go to and hit it. Instead of having the patience to find the hole and understand how much longer they need to wait or if they just need to go ahead becasue the hole isn't going to develop. Emmitt was so great at understanding that and really, it was an underappreciated talent he had for it.
People thought the O-Line 'made Emmitt', but it was really the other way around...Emmitt made the O-Line look great because he had such a great feel for when to allow the blocker to get out in front or when to say 'screw it, gotta hit the crease now or I won't gain any yards.'
Obviously, it's a tough skill to develop. So high schools and college teams would rather see their talented running back use their athleticism.
The problem that Alex Gibbs started to run into is that he fell in love with smaller O-Linemen to run his scheme and they were great at the run blocking, but they were small and when teams would rush the passer, they struggled at times to deal with bigger 3-4 fronts.
YR