If you lower the price too much, you get a different sort of clientele. The sort of person who wrecks the place and annoys all the other patrons nearby.
Then the cleanup costs a lot. Often more than the amount of revenue collected on the room.
It absolutely makes more sense to keep the hotel room empty than to lower the price to keep it fully occupied.
If it is free to show the movie then there is no penalty to running extra sessions. If it isn't free, someone is being paid. If that is a different someone to where ticket money goes they care more about sessions than viewings.