Most basic idea: back in USSR times 20000 tanks steamroll europe following heavy artillery bombardment, and 80000 mechanised armour units.
Implementing that on much smaller scale turned difficult... if not impossible
Russia had 1500 units of heavy armour in usable conditions, in active units. They lost 500 as a minimum according to Oryx, and those were the most up to date vehicles.
Russian tank forces were never made to work without their heavy self propelled artillery which pounds treelines, and defilades with "walking artillery barrages" in front of them.
Why they can't deploy arty en masse as USSR did? Not enough arty officers, and logistic trains can't sustain real "walking barrages" today.