Why are adult fleas wingless?

Adult fleas are wingless because they lost their wings during the course of their evolution as parasitic insects.1

While they no longer possess wings, fleas have retained certain components of the ancestral flight mechanism, specifically a hinge made of a rubbery protein called resilin. In flying insects, this hinge assists in wing strokes; in fleas, it is located on the thorax where the legs attach to the body. The resilin pads store energy when the flea crouches, which is then released to power their highly developed jumping legs. This adaptation allows them to leap great distances, effectively enabling them to "fly" with their legs.1

The loss of wings is a common secondary adaptation among various groups of insects, particularly those that have adopted a parasitic lifestyle, such as lice and fleas.2