Yes, I am interested.
Well I will see if I can find some research on the subject. But just off the top of my head think about what a tail is used for. Quadrupeds primarily use it for balance, it is also used to keep away pests. New world monkeys have prehensile (gripping) tails, but old world monkeys don't. Some animals use them for comunication and display.
So we, and the other great apes are bipeds. We don't really need it for balance. Our arms and hands are free to swat away pests so we really don't need it for that either. Ok so maybe you still think it would be useful, but consider the disadvantages as well.
If you had a tail you would have this extra appendage that you don't really need. Something else to get caught in things, something else a predator could grab onto. It would be something else that could get cut, hurt, infected. Can you really think of any advantage that would outweigh those disadvantages? For a quadruped or a tree dwelling animal sure, but not for an ape.
So our ancestors with longer tails may have been caught by predators more often. Those with smaller tails would have been selected for. It wouldn't take too many generations for the tail to disappear altogether.
Does this explanation make sense?