Why does the spinal dura mater not have two layers like the cranial dura mater?

Periosteum: The outer layer of the cranial dura mater derives from the periosteum of the inner table and it is not tightly attached to the skull. It can be dissected away from the skull and separated from the inner layer of the dura. In the spine, the periosteum attaches tightly to the vertebra and the epidural space is between that and the dura mater.