User blog comment:Master Sima Yi/Assassinews 05-08-'14 – Assassin's Creed: Rogue announced/@comment-3046679-20140910233354/@comment-13471981-20140911064922

I honestly can't think of a reason besides the fact that they want to de-emphasize just how many entries there have been. If numbered, we would've had Assassin's Creed 5: Unity and Assassin's Creed 6: Rogue on our hands - granted, that doesn't sound too smooth either.

I kind of don't expect to see numbers return, though perhaps they will go with "Assassin's Creed 5" if the next one is set in a radically different time period. After all, both Unity and Rogue are still in the 1700s.