How many kinds of cicadas are there? That depends on what you mean by kinds.
There are over 190 species in the U.S.A. and over a dozen in Canada. Australia has hundreds of cicadas and you can find a list of them here.
World wide, there are over 3,390 species, with new species described every year (I counted them all in this book)!
A good source of papers about new cicadas is Zootaxa.
Any research on Why there are so many Species of Cicadas?
Compared to other insects, there are relatively few genera and species of cicadas.
Looks like there are ~17,500 butterflies, ~12,000 ants, and ~350,000 beetles (wow!)
There are a lot of factors that can cause an animal to develop into a unique species. In the case of cicadas: the variety of tree and host plant species, soil types, local climates, climactic events, geological events, and predators all play a part in causing cicadas to separate into different species.
This year I heard one or two Magicicadas in our yard in May, and the usual yearly cicadas started singing in June which is VERY early! What’s going on?
Thank you for counting them! Very useful number. That is more than I had thought were described.