Why do Herbivores have many different types of digestive Systems? for example a Rabbit has a mono-gastric Digestive system and a hippopotamus has a pseudo ruminant digestive system but they are both Herbivores.
This is kind of like asking why animals have so many different ways of moving quickly (dogs run, birds fly, kangaroos jump...) There is a problem, and evolution has solved the problem in different ways.
In the case of herbivores (and in particular leaf and grass-eating herbivores) the problem is that their food is low in nutrients and high in indigestible cellulose. The particular solution depends on other evolutionary pressures. For many leaf-and-grass eaters the solution is to become big, with complex many-chambered digestive systems that allow for long slow processing of the food. Horses, hippos and ruminants all tend to be large. This means that the animal can eat a lot and hold the food in themselves for a long time, allowing enzymes and gut bacteria time to work.
Rabbits, as burrowing creatures, can't evolve a large size, so depend on a range of gut bacteria and coprophagia to pass food through their intestines twice.
Grasshoppers can produce their own cellulase to break down cellulose without microbial assistance.
There's more than one way to do it and as a rule, in evolution, for every possible solution, some animal will be using it.