I'm sure someone can find something a bit more specific but here's a great answer from u/cleopatra_philopater that is focused on language but I think you can get what you need from it:
Starting in the 4th century BC Egypt was ruled by Greeks, and then Romans(/Byzantine empire), for nearly 1000 years, becoming first a center of Christianity and then Islam.
As you can see in the answer anti-pagan measures undertaken by Rome led to the closure of temples and the loss of priests who could read and write hieroglyphics. Over time Greek would become the lingua franca (and then Arabic).
One more thing I'd add, though maybe not your intent "decline/collapse" creates the wrong mental image. Egypt changed over a millennium of external rule, but for that whole millennium it was wealthy and powerful, a trend that continued into the Islamic period.