I've kept many fish over the years. After so many, you just realize what you can afford to keep and/or determine that you like certain personalities/traits in certain fish. You can't really say that CA/SA is "one type of fish" since there are so many variety in size, temperament, care requirements, etc. Same with African Cichlids. It's still pretty broad. If you're talking about someone keeping a tank that is purely Frontosa or all Endlicheri or just a tank filled with nothing, but Red Cherry Shrimp, or even a tank with a single Dovii, there can be many different reasons why. Dovii are probably one of the most aggressive Cichlids and tend not to play well with others, Red Cherry Shrimp will more likely be eaten by anything, but small fish, a pure Frontosa tank can make a stunning display, but I haven't really seen people who keep Polys with just strictly Poly in their tanks (unless for breeding purposes).