As I understand it, that's not of what most of the backlog consists. It is important to remember rape kits are merely one form of evidence and resources are limited. In cases where the perpetrator admits guilt, the time and cost of running them is unnecessary. In cases where other evidence exists and is sufficient to convict or get a plea, the kit need not necessarily be run. In cases where it is determined no rape occurred, or charges are dropped due to the victim recanting, there is no reason to run the kit. 

Again, going from memory, that's what most of the backlog is. It isn't primarily kits in active investigations where DNA will be key that aren't getting run. It's prioritizing them that leads to the backlog.