Because every fact pattern is different and because touch DNA can be extremely misleading. All sorts of tiny, innocuous direct and transferred DNA can turn up using trace DNA. JonBenet, her family, the people she came in contact with did not live in a vacuum.
The DNA of 6 unknown people turned up on/about JonBenet's body - 5 male and 1 female. I don't think six people assaulted and killed her that night and a single killer alone would have left far more than a few cells given all the activity that took place in the kitchen and the basement. Further, as was demonstrated last night, there was no way a single intruder came through broken window without leaving far more evidence, and the ransom note was over-the-top staging to deflect away from what really happened.
It was an inside job.