SAP is a systematic program, it knows values that are present in the PO, but it does not just look around to see if it can find anything in the vendor or in the material.
you can classify a material multiple times, one characteristic can be used in several classes, so from which class should it take the classification?
so you have to help SAP a bit to make it know the values to be used.
if the material is configurable then you do this configuration in the PR or PO, and then SAP knows the characteristics.
Without this, you would need to determine those characteristic values via a user exit