Because I don't remember much of it. Most of it was just 'this guy was in the conspiracy against Ezio's family, kill him' for like a dozen dudes. Pretty boring and repetitious, to be honest. That modern day stuff was okay, from what I read on Wikipedia in order to remember (and have subsequently forgotten: can't have been great, then), and I know ACII's gameplay was a cut above ACI's (much more varied with more activities, assassinations, story mission setpieces, etc.), but people praise its plot as if it's the pinnacle of the series. It's not. ACIV's plot now blows it out of the water, IMO. I should probably reread what Brotherhood was all about now too. Something something Cesare Borgia.