That's because the developers never intended for you to fight off whole groups of people, the mechanics were designed in a way to push you towards playing the game a certain way, you're meant to be stealthy and avoid large scale fights.Well you could slaughter everyone in AC1 also but there was a terrible mechanic where you'd assassinate somebody and neverending respawning dudes would fight you endlessly unless you escaped.
That's why I expected more of a "hitman" type of experience in different historical settings from the series.