Why did the Pythagorean brotherhood use a 5 pointed star as their symbol?

Since the five-pointed star and the six-pointed star hold significance in sacred geometry, I would believe it refers to the divine mathematical points of a human being's body;head, hands and feet. The five points are symbolized with a star. These points are also very important to any of the ancient adepts of esoteric knowledge, since our head, hands and feet also are the pathway gates of bioelectric, energetic currents. The six-pointed star refers to the merkaba or star tetrahedron, the human lightbody.

Read "Isis Unveiled" By H.P. Blavatski and "The Ancient Secret of the Flower Of Life" By Drunvalo Melchezidek for more information on esoteric studies and sacred geometry.