Why was the period after the collapse of the Mycenaean civilization called the Dark Age?

Answer 1

Basically because the Mycenaean was a period of comparably high development. As the Mycenaean civilization ended the Greeks abandoned living in large scale settlements and lost the use of their linear B script.
The period is called a Dark Age because it does not yield much archaeological or historical evidence and a culture which was once advanced had in many ways regressed to a more primitive state.

Answer 2

Because when the civilization collapsed it took out the power lines and they we're left in the dark until the electric company could get out there and get the poles back up and running.