Assassin’s Creed as a series already has 14 years under its belt. In all those years, new adventures have steadily appeared, gradually changing the series. Navigating a pirate ship through the Caribbean? That was unthinkable at the start of part one. But Black Flag is still one of the most popular games in the series.
Logically, with so many titles and so many years, some features and mechanics have been revised or even completely dropped. The latter is particularly unfortunate because some ideas can never develop their full potential.