that and poor people and people of color can't exactly reach out to the police or other institutions for help as readily. so we're easy targets. same reason there's such a high rate of child abuse towards indigenous kids and kids with disabilities.
i think part of why it seems this might be occuring more often is just that it's becoming more visible - which i believe is a really good thing (and i work on some initiatives to make bullying and other forms of violence more visible in my work with youth). in order for anything to happen, people have to first be aware that there is even a problem and it's one so big that it can't be ignored anymore.
http://www.generationfive.org/ is an amazing org that's working to end child sexual abuse. they're definitely worth checking out.