I was wondering the same thing myself after Trent Richardson is looking more and more like a bust. I think it's mostly because they have so much great talent across the field. They run behind the best OL's year in and year out in the country, so that in itself will make a decent RB look great. Add to that very good wide recievers and a game manager QB, and you have a very good RB, at least in college.
I bet you will start seeing Alabama RB's draft stock dropping over the next few years, as well as the LB's (same principle as playing behind a great O line, except on defense) and the QB's as well, even though they have always been game managers at Bama and that's it. I'm not saying the RB's, LB's and QB's suck at Bama, but there performance is based much more on the guys around them than there own talent. That leaves the O Line, D Line, WR's and DB's as truly great players at that particular program.They depend on there own ability much more than the others mentioned above.