It is because under a PD, you can mandate certain restrictions. Example here: workforce housing.
There is are no provisions under State Law that allows municipalities to force workforce housing in general zoning.
However, we are a slightly limited Home Rule State...since there are no provisions that allows, likewise there are no provisions that say a municipality can not force workforce housing.
The issue boils down to politics. No municipality has passed regulations forcing workhouse housing for they do not want to be sued over the matter and lose in court.
They want the State Legislature to pass a bill that explicitly allows it first. However, those bills are not welled liked by rural districts.