Why does Earth have plate tectonics while Venus keeps a single lid?

Convection with strongly temperature-dependent viscosity generically produces a stagnant lid. Identify the lithospheric weakening mechanism (water, grain-size reduction and damage, pore fluids) and the planetary parameters (surface temperature, water content, mass, interior temperature) that place Earth in the mobile-lid regime and Venus outside it. An answer is a regime diagram from a rheology calibrated on experiments that places both planets correctly.

Three-dimensional spherical convection with lab-calibrated weakening rheology that reproduces plate-like Earth and lid-like Venus with no tuned parameters.