It's a fast-twitch thing. There was an interesting study done at the '68 Olympics. They had a bunch of athletes compete in OTHER sports just to get some test data. One interesting thing was that the olympic lifters (even the really huge superheavyweights) had a much faster 10-20 yard burst than even the guys that trained in the 40. They didn't have the technique for coming out of the blocks, but because OL training emphasizes quick burst/explosiveness, they could FLY out of the blocks. And then totally run out of gas.
For me, the 40 time is interesting to look at but for a RB, I'd prefer having that initial burst into the hole. Most RB's don't break a 20+ run more than 10-15 times a year. If you can average getting 4-5 yards a carry but only have a max run of 15 yards, I'd prefer that to someone who breaks a 40 yarder every 30 touches and only gets 2-3 yards per carry on the other attempts. So running for 40 yards is usually irrelevant and unnecessary for a RB.