Why did Egypt, a big granary in Roman times, become a grain importer now?

Not only the population problem, according to archaeological research, the climatic conditions in ancient Egypt are more suitable for agricultural development than now. It is said that the Sahara desert at that time was not as big and dry as it is now, and the annual flood of the Nile can also bring fertile soil. Ships were also popular means of transportation at that time, so the climate was still very good. However, with the continuous expansion of desertification and population growth, it has become an importing country.