I agree with the posters saying players are stronger and faster. Do you remember Roy Williams blowing up Emmitt Smith?
There is an article, "How ex-Cowboy..." you can google it. The article talks about the players nowadays going for "killshots" Tackling has taken a step back, and players go after hard killshots.
Add that mentality to the faster and stronger player and you can see why a RB doesn't last long.
For those who think players aren't faster and stronger. Take a look at the overall conbine numbers since they began recording them. The 40 times, the bench press, the broad jump, the shuttle they have all improved in every single position category in the past decade, and player's weights have gone up in most position groups also.
As much as we want to wax and wane about the "good ole days" those days are long gone.