Every reason has already been listed as to why, obviously.
However that been said they need a lot more training to become good sprinters, they can run very quick naturally but their form is clumsy and slows them down, mostly in the arm movements.
However white sprinters i.e. Have had to better their form in order to become quicker, and can adapt that change a lot easier, I can run 11.2 but once ran 10.87 for 100m (people assume that this is quick but remember on game day it depends how you run, and the milliseconds over are what counts.) However with the right training they become quicker in a flash.
I've noticed people who play football tend to have a really good top speed but it slows them down in sprinting competitions, I myself play and I tend to set my hips lower when I run as it equals better exit angles to keep acceleration when turning.