User blog comment:Master Sima Yi/Assassinews 19-03-'14 – Two Assassin's Creeds for 2014?: Unity and Comet/@comment-5019018-20140321153336/@comment-1379990-20140321222308

I think they aren't using Roman Numerals because they were using them really for Desmond's story and Black Flag was still part of that. They did say it was to serve as a bridge between the old story (Desmond's) and the new one. And since there's another game is coming to current gen (according to rumours) it would be confusing to use roman numerals, I guess.