For many members of the public - especially those who may feel on the fence about the ethics of keeping animals in captivity - safari parks may seem like an idyllic answer. There's obviously the element of space. An enclosure of fifty to a hundred acres in size hardly constitutes a "cage." The chance to display larger, more natural social groups is obviously desirable, as it leads to more expressions of natural behavior. In every respect, safari park settings seem to more closely resemble the wild. Heck, there were moments when the San Diego Zoo Safari Park seemed more "African" than the actual African national parks I'd visited did.
There are, as you can imagine, also plenty of challenges to maintaining animals in these conditions. Most have to do with space. Sure, you obviously need a lot of space to pull this off, which is why many American and European zoos - built in the center of cities in the 1800s and 1900s - don't have the room. From an animal welfare perspective, however, it's very difficult to manage animals when they are in enclosures so big that you can barely see them. How do you make sure everyone is getting enough to eat (especially if they are a part of a big herd?). How do you monitor for signs of illness or injury if you can't get close enough to observe carefully? Heaven help you if you need to catch an animal up and it's not inclined to let you do so. Bigger exhibits are hard to ensure the safety of, as well, and the longer the fence line, the more likely that a wily predator will find a gap.
Big social groups can also lead to problems. For one thing, there is an increased likelihood of bullying. Another challenge concerns breeding programs. For the purpose of creating sustainable populations, zoos want to promote genetic diversity - and to do that, you need to know who has been breeding with whom. If you have a large herd, it can be difficult to say for sure which sire and which dam produced which offspring.
The climate has to be suitable for the animals in the park also, since the animals are maintained outside all the time. San Diego's dry, warm climate works well for East African ungulates, while East Asian species do well at The Wilds in Ohio, or the Smithsonian Conservation Biology Institute in Virginia.
Not all species work well in safari park settings - some are simply delicate and require closer care than can be provided in those enclosures. At San Diego, for instance, the okapi, as well as many of the smaller, more skittish antelope, live in their own enclosures. Here, keepers can be sure that they are getting a proper diet (not bullied by exhibit mates) and are easily accessible for care.
In safari parks, it's more of a matter that certain species and certain facilities do mesh, combining to create an exhibit and an experience that few guests will forget.