Energetic electron depletions are a notable feature of the nightside Martian upper atmosphere. In this study, we investigate systematically the variations of the occurrence of depletions with both internal and external conditions, using the extensive Solar Wind Electron Analyzer measurements made on board the Mars Atmosphere and Volatile Evolution. In addition to the known trends of increasing occurrence with decreasing altitude and increasing magnetic field intensity, our analysis reveals that depletions are more easily observed when the ambient magnetic fields are more horizontally inclined and under lower Solar Wind (SW) dynamic pressures. We also find that the occurrence increases with increasing atmospheric CO$_2$ density but this trend is restricted to low altitudes and within weakly magnetized regions only. These observations suggest that the formation of electron depletions is two folded: (1) Near strong crustal magnetic anomalies, closed magnetic loops preferentially form and shield the atmosphere from direct access of SW electrons, a process that is modulated by the upstream SW condition; (2) In weakly magnetized regions, SW electrons precipitate into the atmosphere unhindered but with an intensity substantially reduced at low altitudes due to inelastic collisions with ambient neutrals. In addition, our analysis reveals that both the ionospheric plasma content and thermal electron temperature are clearly reduced in regions with depletions than those without, supporting SW electron precipitation as an important source of external energy driving the variability in the deep nightside Martian upper atmosphere and ionosphere.