same reason India and china cant mobilize their 20 lakh troops on just one front line. They are needed elsewhere to fight their own battles , Russia is the biggest country on earth by a huge margin, they have plenty of conflict zones.
if china was invading tiwan ,it wouldn't call its troops from eastern theater and leave themselves defenseless with India, now imagine how Russia feels surrounded by nato and their lapdogs.
Also last i checked even propagana westren sources say russia has used only 135k troops, the total pro russian troops is around 180-200k , made up of separatists , militants and volenteers