Because the vaccine is most effective when administered to virgins. This isn't about childhood behavior, it's about the fact that any woman who has sex at any point in her life is potentially at risk of a preventable virus that could cause a cancer that will kill her.
I understand your point about vaccinating females for HPV before they become sexually active, but why vaccinate little girls a decade before they likely will be exposed to the virus? I guess what I was trying to say and didn't say very well was that I wouldn't want to have to explain to a little girl or the rest of the family why she was being forced to have a vaccination for a disease frequently associated (rightly or wrongly) with sexual promiscuity. If this vaccination is a good thing for every woman to have, there's plenty of time for it later.
Although I don't question your motives at all, I have a sick feeling that there may be more to this movement than disease prevention. Something just doesn't compute.