This is the hospitals way of trying to keep costs down. If they could nurses would be working 60 hours/week. They could cut the staff in half and pay out less benefits.
It costs a lot to train a nurse. Orientation, mentoring, learning the systems unique to the facility...many places don't want to make that kind of investment for a part-time nurse that may later abandon their position for a full-time job with a different employer. However, the number of PRN/part-time positions will probably increase as the shortage worsens. You can always apply and them mention during your interview you're looking for part-time work. If they want you, they'll work with you.
Not sure where you live, Centura has lots of PRN positions in Colorado