There are several obvious explanations for this. Pragmatically, small development teams have an easier time writing, editing and testing simple rules, and narrative games are almost exclusively small shop productions.
More significantly, complex rules tend to reward players who can master the system. The players, in an almost pavlovian reaction, will rise to this challenge and manipulate the rules to their advantage. This creates a feedback loop that inevitably drives the game towards tactical play. Unfortunately, this also tends to distract from the story.
The concept is simple: if you're designing a story-focused game, the game should reward story focused play. Most game designers recognize this, at least subconsciously, and they tend to shy away from rules-heavy mechanics.
But I think there is an even more important reason to avoid complex rules. Complex rules typically place a heavy burden on the GM. The more complex the rules, the more preparation you must do. Before the characters can challenge the Ogre Chieftain to a game of Brokk'Tak, you need to gen up the ogre's stats. If you want to have a mad chase across rooftops, then you need to look up the rules for moving across difficult terrain, balancing, jumping and falling (and probably other things as well). You probably want to take notes.
This creates three problems. First, if the GM invest time in creating a scene, he is more likely to force the scene down the players' throats--railroading them if necessary. Second, as a GM you never remember to pre-generate everything you need. Even if the scene goes exactly as planned, sooner or later you will find yourself digging through a rule book (or just making shit up). Finally, if the characters go off-script, the GM often has real trouble improvising.
Again, the GM should not create the story. He does not write the plot, then merely let the players experience it as they go along. No, the plot should evolve naturally at the table. It should be a collaborative effort between the players and the GM. For this to work, the system must not merely let the GM improvise, but actively encourage improvisation.
As a GM, I want to introduce NPCs at a drop of the hat. If the characters attack my social-skill-statted courtier, I need to keep the scene rolling without batting an eye. I need a system that lets me come up with reasonable difficulty numbers and modifiers without resorting to charts, tables or high school math.
Sure, some rules-heavy games try to help out. Many provide character templates, allowing the GM to more-easily improvise--provided he stays within the scope of those templates. Others use simplified NPCs.
Buffy the Vampire Slayer et. al. take abstract NPCs to the extreme. Almost all the crunchy rules are kept firmly on the player's side. The GM never even touches the dice. When a player tries to stake a vamp, the player rolls against the vamp's defense number. Likewise, when the vamp tries to bite a piece out of someone, the player makes a defense roll against the vamp's attack score. All things considered, this is a pragmatic compromise. After all, the players have less to worry about, giving them more time and energy to focus on rules. Indeed, managing the rules may help fill some of the downtime while others are in the spotlight.
But what should a Story-hungry GM do when he's stuck in a rules-heavy game?
As I've mentioned before, I want to like White Wolf's games. I have many fond memories of old WOD. Whenever I pick up one of the new books, I'm pleasantly surprised. The back stories are intriguing, and the systems seem to encourage interesting characters. Most importantly (to me), I can always find players for a White Wolf game. Sadly, it's often hard to convince people to try indie games.
So, I want to like White Wolf, but--damn--those games have a lot of rules!
Want to know a secret? Mostly, I just fake it. My NPCs succeed or fail based more on the dramatic necessity of the scene, than on the actual rolls. I don't like it, but it's true. Shhhh. Don't tell my players! I do it for the story, but I always feel guilty. I know I'm breaking a social contract with my players. After all, they expect the world's physics to operate as advertised.
But, really, what choice do I have?