When you have a child of that age who exhibits physical signs of sexual contact (in JB's case, there was the eroded hymen, exposure of the vaginal rugae, unusually large vaginal canal which indicate chronic abuse, as well as the blood on the forchette, which is a part of the vagina, blood in other areas of the vagina, bruising of the labia and blood wiped from the thighs and pubic area , all of which are evidence of sexual contact that happened that night) it is apparent that someone with regular, private access to JB was abusing her. The chronic aspect of the abuse was not a one-time event. Kids that age are not in situations where that kind of abuse can occur unless it is with someone who is repeatedly with the child alone.
Like it or not, until the case is solved with a known, NAMED killer every person in the home that night must be considered a suspect in the events of that night, and every family member, including parents and siblings, must be suspect in the chronic abuse.
So to answer your question, it isn't because he was male- females can be abusers, too. It is because he was THERE to have committed the abuse. There need not be evidence of child *advertiser censored* and there need not have been molestation of other children. In fact, often in an abuse situation, only one child is targeted, for a variety of reasons. If it was JR, it was more of a situational abuse, and not true pedophilia. He wasn't attracted to ALL little girls- just THIS little girl, because she was presented as sexualized 20 year old.
SOMEONE in that family was abusing her. Until that person is exposed, JR will continue to be a suspect. There are at least three other suspects. Patsy, BR, and half-brother JAR (who was frequently in the home, as he attended college right in Boulder).