On a public lake you need the following for course skiing.
1. No other boats moving, for several hours, on the entire lake
2. Favorable winds/weather
3. No boats in approximately 2000' x 300' or so of water
4. Course not destroyed
It takes a half hour to hitch up and drive there, maybe 2hr for 4 people to ski including getting stuff on, falls, putting out to the starting spot. At 6:00am. So that entire lake's capacity for slalom is 4 people for the entire day! Most people won't do all that work and even if they do, they may not have time to fit in the rotation.
Private lake may still only have capacity for about 4 skiers before work, 4 at lunchtime, 4 after work. 12 skiers per day assuming everyone is at least somewhat working a day job.