Why did the power of the Egyptian Pharaohs decrease over time?
The power of the Pharoahs decreased as Egypt suffered invasions from foreign powers which the Pharoahs were unable to prevent. Following the death of Alexander the Great (who had had himself crowned Pharaoh,) the Egyptian Empire was considerably smaller than in previous years. Alexander's general, Ptolemy, created another dynasty which included Cleopatra; but at this point Egypt was overshadowed by the growing power of Rome.
The Northern and Southern Kingdoms of Egypt were originally united under the legendary Pharaoh Menes. It was Menes who founded the first Dynasty and was considered the earthly manifestation of the god Horus. He was also believed to be responsible for the flooding of the Nile, which was vital to Egyptian civilization.
Over time, several parts of Egypt broke away from central control and declared their autonomy. During the Middle Kingdom, (2040-2640 B.C.E.) the Pharaohs were able to exert some control over these areas, but never to the extent that had previously existed. Invasions from the Hyskos, who used bronze weapons as opposed to the wooden weapons of the Egyptians, caused resentment among powerful Egyptian nobles who caused a revolt. The Hyskos were eventually expelled, but again with a loss of power to the Pharoah. Ultimately, about 700 B.C.E. Kushite and Assyrian armies invaded Egypt. The Kushites were eventually expelled; but the Assyrians ruled Egypt for almost 100 years before their own empire collapsed.