In 1967 Lamar Hunt came up with the name “Super Bowl” for the NFL-AFL Championship game. The following year, there was much talk about what the Championship game would be called that year. IIRC it was Pete Rozelle who came up with the Roman numeral idea. Most fans thought it not such a good idea, but it stuck anyway.
The first Super Bowl to use the Roman numerals officially I believe was Super Bowl III.