You might ask, “why were their once cicadas in my area, but now there are none?” There are a number of reasons why cicadas might die off in a particular area, or go totally extinct.
1) Destruction of host trees by a blight or destructive insect infestation. Tibicen bermudiana went extinct in the 1950s in Bermuda because of a cedar tree blight. Emerald Ash Borer insects are currently devastating Ash trees in North America. Ash trees are a favorite tree of Magicicada cassini, in particular.
2) Destruction of host trees by humans. Consider that every time a forest is removed to make room for another neighborhood, factory, strip mall or highway, the cicadas that inhabited those areas died. Each time the human race expands, the cicadas must decline. The paper The Distribution of Brood Ten of the Periodical Cicadas in New Jersey in 1970 by John B. Schmitt documents the reduction of cicada populations in New Jersey, the nations most populous state. Also, the entire Brood XI went extinct in Connecticut as of 1954.
3) Extreme weather such as tornadoes, hurricanes, and flooding can destroy cicada habitat. While there are cases where cicadas were able to survive some pretty horrific weather, if trees are destroyed, or grasses that are hosts to young cicada nymphs are destroyed, or if floodwater sits too long, the cicadas are doomed.
4) Pesticides. It should be obvious that pesticides will kill cicadas.