I have to agree with Spinner a lot. More and more art mediums are going on to leather nowadays. To include skulls. I think to understand "why" skulls you would have to understand what they mean or represent to the individual. Just ask anyone that carves them and they will tell you. Chances are that they use them because quite frankly they look 'cool.' Why does someone tattoo themselves with tribal art or flames or other art. Why do some pierce themselves with rings or studs in what I would consider an abnormal spot? Like Spinner suggests, it is just culture or generational trends. I guess when I am 80, I will not have to worry what my skull art will look like on leather instead of my body!
Good luck.