Rolling in park

For the same reason why it will not move now that the half shaft is apart. The differential in the trans-axle will not allow it. The front axles are not connected as a solid unit, there are gears inside the case that allow each side to move independently. to allow the tires to rotate at different speeds during turns. Two gears are connected to the axles. Two more are connected to the ring gear. When you put the transmission in park a locking pawl moves and locks the internal parts of the transmission so they cannot turn. If both axles are good this will prevent the vehicle from moving. In your case the broken shaft allows the gear it is connected to the freedom to rotate because it is not connected to the tire any longer. This is a video showing a rear differential but it works in the same way as yours, just without a driveshaft.

https://www.youtube.com/watch?v=WAyD9HIMtcU