Why are the best index fossils from species that only existed for a short period of time ?

Because once they were very common they disappeared after the mass extinction event. ... Many geologic periods were defined by events that made natural boundary to the period.

i think the miocene epoch spans the time from 23 million to 12 million years ago.

sorry to break it to ya but no one can answer your question it's just

jk but i am serious about the part that most people cant answer it beacuse you did not mention the section you are talking about. but it would if you added some detail and were a bit more specific.