Why didn’t the 757 have manual-reversion capability?
When you have controls designed to be operated manually, in an airplane that heavy and that fast, you have to include things to provide aerodynamic force compensation like servo tabs and offset hinge lines (look at the IL-82's rudder hinges; like a B-29's...the most obvious tell-tail of manual controls on a large aircraft) to make the control forces reasonably manageable, and even there, I can say with pretty much total certainty the IL-82 required muscles the size of fence posts to operate comfortably.  A jet like that with B-29 flight controls is hardly a leap forward.  You also have to mass balance the surfaces because there are is no flutter damping capability afforded by the mechanical control linkages.  All rather heavy.

When you have hydraulic controls, you can get away without mass balancing the surfaces, without servo tabs, and without offset hinges, because the hydraulic actuators (plus flutter dampers in some cases) provide all the required force, AND the required flutter resistance.

So, if you are going to design a hydraulically boosted system with manual reversion, you have to incorporate some of those aerodynamic force aids to be able to operate the controls manually.  But once you have a design that is sufficiently robust from a design/risk/failure tolerance perspective to operate with hydraulics only, there is no point in including manual reversion.  It's a step backwards, adding all kinds of complexity and weight for no benefit.