Question

no sound is heard on the moon because there is

Answer 1

because there is no medium

Answer 2

The moon  has no  air, so for all practical purposes there's nothing to transmit waves. If an astronaut drops a tool on the moon, he won't hear  anything because , again, there's nothing to carry the vibration to his eardrums. That doesn't means on the moon  is completely impossible, however.
So that's why we can't heard sound on the moon