It is because that is the way God chose for him to translate. JS was not an Egyptologist, nor was anyone around 1824 able to read and translate Reformed Egyptian. So God gave JS the instruments and the revelation to do the work. The result was a 531 page book, containing the history of God's dealings with people who left Jerusalem around 600BC and ended around 420AD in Mexico City.
The last time I looked it ranked in the top 10 books ever published, with distribution in the millions, and in 150 different languages.
So, God used a 20 year old farm boy to translate a second witness that he is the Christ, the Savior of the World. As Jesus says as recorded in the bible says: And Jesus answered him, saying, It is written, That man shall not live by bread alone, but by every word of God (Luke 4:4). God wanted you to know His Words to these people and their testimonies about Him. It is as important to know their testimonies as it is to know of the testimonies of those living in and around Jerusalem. Every Word of God is important. Even the Word of God given to JS.