it's because I give a fuck.
more serious, the middle finger is in the middle of your hand. if you lift your hand or arm, it's easier to press the key with the middle finger than with the index finger, because...
if you have a stick, you want it to be straight, so you can poke better with it. if it branches to the left or right, you can't apply as much force because the energy has to split at the branch. and it's basically the same with your fingers, it's just easier to lift with the middle finger.
if you don't lift (like a beginner), then you use the index finger, because you can move the index finger without moving your hand to much, contrary to the middle finger who is much less mobile.
and now I hope that somebody with proper english tries to explain it better than I could ;_;