Why do christians think that showing too much skin is wrong....?

Because most of American Culture was origianlly based on a religious, northern European framework. To a Englishmen of the 17th century skin was equated with uncivilized natives. The idea that if the skin was out there all day every day everyone would simply get used to it and move on never occurred to them. Also skin was and is still ociated with because the more skin a girl shows the more it get's guys attention. One of the things I have discovered over the years that if the religious leadership in this country had their way girls would still be wearing ankle length dresses with long sleeves and several layers of petticoats underneath.