I would hypothesize that due to the unique nature of Egypt. The environment was able to preserve a lot of papyri which preserves a lot of information that would have gotten lost in other climates. So we get stuff from more ordinary people (or more non-elites), administration documents, lost literature.

The flipside is that Egypt is such an odd bird and while we have much more information about this province, we can't be certain if this would apply to other provinces or was unique to Egypt.