Why didn’t the space shuttle have flight control manual-reversion capability?
The biggest reason (as is typically the case with space craft) was likely weight. It costs quite a bit of money to get even small amounts of mass into space, when talking about a vehicle keeping it light while maintaining a basic level of safety is key. 

Unlike the 737 the shuttle lacked the yoke style controls that would be required for this kind of a system. The shuttle stick not only operated the flight control surfaces while on approach but also drove the reaction control system while in space. So there would be no way to operate a manual backup system even if there was one (without changing all the control surfaces). 

The orbiter also had a triple redundant hydraulic system which was considered fail safe enough, and due to the highly automated flying of the orbiter hydraulic systems proved to be what was needed to make the system a reality. Cable controls for manual reversion in a flight regime barely flyable by humans would have largely been dead weight.