There are a thousand-odd cultures, societies, or lands that faded away....each for a different reason. In the cases of most....they simply transformed into something more or something lesser.
In the case of Egypt’s Kingdom....they needed to be able to take on any invader or threat, and they simply didn’t have the power to ensure that.
I also think commercial power (banking, trade, wealth, etc) eventually grew to be a weakness in their case. Rome stayed in a strong state as long as they were the central theme for all trade in the region. When they lost their ownership and control....they weren’t able to stay in control.
the rest of the world changed
the old kingdom did not
it succombed to exterior change