Many species, or at least the region they live in, need a relatively narrow set of environmental criteria to be true in order to survive. For example, if an alligator in the Florida wetlands experiences a long-term drought, the plants that the smaller animals eat can dry up and those animals die. Then the alligators have no animals to eat so they could die. If this was to happen over a long period of time and on a worldwide scale, a species could become extinct.
I hope that helps.