Rudder Shudder
To jump from Fail Passive to Fail Active (Fail Op) in existing airframe types has historically meant upgrading the rudder.
The MD-80 has a fail pasive (single) rudder control. When the design engineers were looking at upgrading to Cat IIIb, they looked into a Fail Op cubed rudder control, whatever that means. It didn't fly.
The 737 series did not get Cat IIIb until they came out with the Fail Op rudder system, which coincided with change to triplex autoland like the rest of the Boeings, I believe. The Fail Op rudder also conveniently cured the hardover drama.
Think about it: what good would a Fail Passive rudder be in a Cat IIIb touchdown and rollout?
GB