Firstly, they are allowed to offer almost any activity they wish to their guests. Night safari is a major attraction, along with mokoro, boating safari, fishing and walking safari.
Secondly, they are able to offer off-road driving, unlike other areas where you are not permitted to leave the roads and tracks. This ensures that you can follow game wherever it goes and get really close to the best sightings.
Finally, a private reserve is exclusive, so there are very few vehicles around each sighting. And although they are to be found elsewhere in Africa, no other country has such large private reserves.