Why did invaders had difficulty conquering Egypt

At the commencement of the Muslim conquest of Egypt, Egypt was part of the Byzantine/Eastern Roman Empire with its capital at Constantinople.

Egypt is a weird land. On side is mountains and the other is water. You would have to cross one of those to get on the land, granted if you weren't seen first. Also, Egypt is hot. Invading it would require lots of supplies for long walks in high degrees. In the end, it was the people of Egypt that really fought invaders off, it was the land itself.