I am not sure, but I think it's probably because he is already serving a double life sentence and it was judged unnecessary to spend further resources on prosecuting him. Prosecutors have that kind of discretion:

If the offender is already subject to a substantial sentence, or is already incarcerated, as a result of a conviction for another offense, the prosecutor should weigh the likelihood that another conviction will result in a meaningful addition to his/her sentence, might otherwise have a deterrent effect, or is necessary to ensure that the offender's record accurately reflects the extent of his/her criminal conduct.