Maybe that is why it became art in the first place, the mystery of the leaves :D

On a more serious note and have to admit that I might remember wrong, but as you probably know, they used to make them with genitals and so forth as sign of virtue etc. But i think it came with the Catholic church that didn't like it, so they chopped them off and might be why they started to cover them up as it was seen as vulgar or some religious thing. So maybe a leaf were the least they could get away with, could explain some of reason at least, but again im not 100% sure :)