A season-ticket holder leaves the car for ten or twelve hours: slow charging is enough, they want their usual bay and they expect to find it free. A short-stay user is there between one and three hours, has no assigned bay and wants to charge as much as possible in that time.
When the car park installs with only one of the two in mind, it fails the other. If everything goes into slow charging in season-ticket bays, the short-stay user finds nothing. If everything goes into the short-stay area, the season-ticket holders do not charge and go to another car park.
On top of that comes the part that is almost never resolved well: billing. The car park already has its own tariff system, and if charging goes through a different channel, the user ends up paying twice in two different places and the operator keeps two sets of books.