Timing, for one thing. "The big one" might not hit the San Andreas Fault for 10,000 years for all we know.
It might not ever hit. Crustal rocks can permanently store tremendous amounts of energy as stress; if the stress is changing, the fault might only move enough to release some small delta of the total stored stress. But: grease the system and you might release a lot more, and all at once.
It's very hard to predict what will happen, either naturally or human-triggered. IMO this is a good argument to not rush it. Every day that passes gives us more time to understand and prepare.
