I think all the theories and reasoning being brought up for its success are all noise, really. When you have to start theorizing why something is successful you're probably just over thinking things. The fact is it's probably just a really good game.
I share similar sentiments towards the Metal Gear series as of recent, 4 really didn't do anything for me, but it doesn't stop me from understanding why others like the game and what makes it such a huge success.
Though I still hold the original game in very high regard, so maybe I'm partial to it.