That's the problem with annualising massive games. Because the studios have to leap-frog each other's release there's no time for vitally important pre-production with a small team. No time to gauge the response to the previous game and the current temperature of your franchise.
I imagine Ubisoft had no idea Black Flag would be so well received and act as the palate cleanser the series needed. And when they did realise? Unity was in feature lock and Rogue was their B-game.
It also doesn't give you enough time to actually make major improvements to your game mechanics.