Well, this subject hits pretty close to home. I have 9 tattoos. I don't regret any of them. They don't change me as a person, nor do they change my faith in God or my belief in Him. I have never been drunk when I got one (reputable places won't even let you in if your are intoxicated), nor have any of mine been infected. They are required by law to used new, sterile needles, and if you take care of it the way they tell you to, they don't get infected.
I have each of my daughters names down my forearms. When people ask me about them, I get to brag about my girls. I have my wife's initials in Celtic script on my left arm. When people ask me, I get to brag about my wife. I have a large cross on my arm. I have a sparrow on my chest because his eye is on me, just as the scripture says. I have a back piece of a man looking towards Heaven. See it and other Christian themed tattoos:
http://www.pathcom.com/~gennaro/Other_Tattoos/Other_Symbols/looking_to_heaven.html
When visible, it is a great conversation piece, and a great witnessing tool.
I will get more, because I am an artist, and I am using my body as a canvas. My wife likes them (yes, she has several). More importantly, they mean something to me. They display my love and commitment to my family, and my faith in God. If anyone were to judge me differently for my choice of expression, they would be in the wrong. If you like tattoos, and want one, get it. If you don't, don't. In reference to the Levitical Law, I believe I am saved by God's grace and no longer subject to it.
Oh, and to answer the original question, it is just a trendy place for girls to get them. Easily hidden, easily displayed when desired.