Basically, the growing use of mercenaries by Greek poleis in the 4th century BCE came about because of the destruction caused by the Peloponnesian War.
The Peloponessian War ended up weakening Sparta and Athens, but not enough to make them stop fighting one another. They were (along with Thebes) still strong enough to fight, but not strong enough to be as dominant as they once had been. This led to a situation in the 4th century in which there was a great deal of fighting. The fact that these wars were so numerous and so long-lasting made it necessary for Greek poleis to employ mercenaries to fight for them.