Mass was originally said in the vernacular from the very beginning of Catholicism. That is how it ended up being said in Latin only for much of history because when Christianity began, outside of Jerusalem, Latin was the vernacular. (While it was likely that Mass was said in Aramaic in the earliest days in Jerusalem, that ended shortly after the Roman army destroyed the city in 70 A.D.) Other liturgies, like Greek and Coptic, also existed from the earliest days.
By allowing the Mass to be said in vernacular in modern times, the Church is returning to her roots and the way it was done from the beginning, when everyone heard the liturgy in the language of the people. While this has been allowed where appropriate since Vatican II ended in 1965, Latin has remained the official language of the liturgy of the Roman Rite.