Answer:
Because Pharaohs were considered God-like people.
Step-by-step explanation:
Ancient Egyptians believed that their rules were semi-divine, like demi-gods, and as demi-gods, they supposedly had power over natural phenoneman like disease or famine.
If crops were failing, or if some infectious disease was spreading, it was seen as a failure on the Pharaoh's part, and only because of they way he might have responded to it, but also because it was thought that he either did not avoid it, or helped cause it.
Finally, besides the superstition aspects, there is the socioeconomic factor to it: if people are sick, and hungry, they are more likely to despise their rulers, and that is something that happens up to this day.