Because there are different sorts of plants and they are cultivated in different climates.
Some plants need a lot of water. When these plants are cultivated in a warm climate the finer material suits them better because there isn’t any unnecessary drainage or evaporation.
Mixtures with good drainage properties are more commonly used in the colder regions. It is important that they assist optimum drainage without creating air deficiency. (The plant root needs water, air and nutrients). In the bottom layer of the medium, where the most active roots are, good drainage properties are particularly important.
The differences in size are mainly to accommodate the installation in different nurseries.