There is a very good reason for Greek.
John, who wrote it down, was in Greece at the time, and Greek was considered one of the ‘learned’ languages even by the Romans, who were the super power of the world at that time.
Greek as a language, and as an alphabet, has changed very little since ancient times.
For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek.
Acts 21:37
Paul Speaks to the Crowd
As the soldiers were about to take Paul into the barracks, he asked the commander, “May I say something to you?” “Do you speak Greek?” he replied.