There are a bunch of factors to note, here.
* The time the girls lay in a harsh environment, exposed to the elements
* The fact that most, if not all the girls would have been skeletal by the time they were found
* The progression of science surrounding DNA since the girls were found. They can get full profiles with rootless hair, degraded samples, mixed samples, minute samples, which was future science or hit and miss in 2010.
We also don't know what he did postmortem to try and erase his DNA from them, such as washing the bodies or using chemicals such as bleach.
MOO