Well, it's really a combination of factors. I'd say that the major ones are the GM's willingness to roll with pretty much anything, as well as the roleplay of the players (I am probably the biggest culprit here since I love torturing my characters ) and premise of the game itself (which also gets pretty dark at times). Although fighting used to be central, it's not so much anymore, and sometimes fights are even resolved narratively.
There's actually two components to any game system: How the dice resolve mechanics (which I will call "hard task resolution") and how whoever is in charge - usually the GM - resolves actions ("soft task resolution"). The balance between each of these resolution mechanics does a great deal in determining how gritty and "crunchy" the game feels. In this case, I would say it is this game's reliance on soft resolution in addition to the fairly minimal hard resolution (2d6 et al) that creates the authentic brand of chaos that is the game's trademark.
I want to talk about this game some more since it is my darling and I love it to death. I am not the GM, but I've been a player since the beginning, so I am very attached to it and a lot of it has been my doing.
In the beginning, we actually started with a much more inelegant system - superpower use was tracked with energy points, everything was rolled for with 1d6, etc. We eventually moved to a less strict tracking system, and the 2d6 task resolution, around the same time we realized that while it was originally a fan game it had taken on a life of its own outside of canon.
The game's premise was originally entirely based on the fights, but as we moved to roleplay instead of combat as the major driver of the game, we refined the system to be much fluffier. A lot of things are still determined by dice roll, especially surrounding power use, but now there is much more focus on soft resolution - initiative and turns were simplified, and on at least one occasion fights have been mostly diceless, only going with what is dramatically appropriate.
I think that the movement to a softer system was to the game's benefit; while randomness still has a role to play, and in fact is a significant driver of the chaos, it is the GM's ability to interpret the dice rolls that makes the game truly
work. Part of good GMing is knowing when to follow the rules and when to break them, and I think our GM is very skillful in that regard. The system is only half of the game, after all.