invertebrates have a very quick life cycle and die very easily, as a result, they evolve very quickly into many different types of animals. also, they are often very small and their ecosystems are subject to much quicker change than that of vertebrates, meaning that mutations in their genes become relevant more often. Many invertebrates span very large areas (sea animals span most of the oceans, insects span entire continents) and so they evolve to be different species in different places.