Well I think because ⲓⲁⲣⲟ alone basicly means "river", but it's also used for "Nile". To specify, the user decided to write ⲓⲁⲣⲟ `ⲛⲭⲏⲙⲓ (The River of Egypt), otherwise it would not be clear wether the Nile or any other river is meant.
In a dictionary, words are defined. (i.e. "Egypt": a country in North Africa). In a lexicon, not only words are defined, but there is also much information about the respective topic (i.e. population of Egypt, a map, politics, demographics, ...)