Revenues are not built or collected in enough abundance from rapists....  this is why many kits are untested on a shelf.. Others may be sitting because of who suspect is (don't want to incriminate the "good old boys'')
     Now that DUI convictions are so profitable you wont find any blood test specimens that are not processed....  talk about follow the money, it seems to be all about the revenue..
    The rest probably sit there because investigating is so hard.