Racks are cheaper, easier to heat, take up less space, and are easy to clean. But they really only make sense if you have a large number of snakes.

As far as enclosures go, imo aquariums just don't make sense. They're heavy, they build up calcium deposits, and they make it difficult to maintain heat or humidity. I am a BIG fan of custom acrylic cages such as Animal Plastics, Boaphile, Visions, etc. They're easy to maintain, lightweight, provide visibility while keeping the snake feeling secure, and are all-around nice aesthetically and functionally.

I think it's simply a matter of balance. Obviously if you have 20 ball pythons (not the best display snakes to begin with) it doesn't make a whole lot of sense having an expensive acrylic cage for each one, but I know many large-scale breeders will have several display cages for their nicest-looking snakes. I've never used a rack system, and especially now that I'm down to four snakes I have the space and resources to keep all of them in elaborate setups.