See, there's one thing it does which no other game/series does.
Historical fiction. (Well, I know the whole assassin thing doesn't fit into the genre, but I mean 'historical realisation/simulation'). Like, imagine if someone told you in 1996 that in 20 years we'd be able to free-run over a near 1:1 simulation of revolutionary Paris's central districts, an area few square km in size, looking almost like real life (crappy NPCs notwithstanding), and chock-full of people? Or a stretch of truncated Carribean sea with several cities, ship battles, etc?
As much as all the games have deeply flawed mechanics and hodge-podge gameplay, they are the only ones doing this exact thing, and doing it so fucking well.
I buy almost every new release (skipped Revelations and Black Flag at release day) just because I love the historical fiction.
I'll be buying Syndicate in the next couple of months (sooner if I see a good deal) because I can't wait to see their London.