The girls then uploaded a video of him being held down naked to Youtube and they weren't prosecuted, why is this?
Because the relevant law enforcement decided not to
In the US (and indeed in all common law jurisdictions), law enforcement and prosecutors have discretion over when and if to lay charges by considering such things as the wishes of the victim, the prospects of success and whether charges would be just in the circumstances.
At best, this allows a measured response to the particular situation. At worst, it enables discrimination and persecution. Either way it solves the problem of allocating limited resources to comparatively unlimited need.
Because no charges were made by the responsible parent.
2011-06-11: Online video shows Fort Myers boy being bullied
Fort Myers Police says the girl would have faced misdemeanor battery charges, but that the boy's mother has refused to press charges. She wants the girls' parents to hand out a punishment instead.