Though not a full answer per sé, it might help to look at the fact that the majority of tanks developed after the mid-1950s introduced automatic loading systems. These auto-loaders had several distinct advantages, namely reducing the crew from 4 (Commander, driver, gunner, loader) to 3 (Commander, driver, gunner/operator), improving reloading speeds and thus the rate of fire, and finally allowing for the design of vastly smaller turrets with lower silhouettes which made the fighting vehicles less visible on the battlefield, improving its survivability in combat. The T-64, designed between 1951 and 1962, effectively introduced the automatic loading system partly as a practical novelty, partly as a practical necessity of the design; the proposed turret of the design was far too small to house 4 individuals including a loader that needed considerable space to move around to reload the gun. All designs from the T-64 up until the T-90 retained this automatic loading configuration. One might argue that the T-14 Armata should be included in this list, but since its crew configuration heralds a new advancement in both layout and design I have opted to exclude it.

So back to the matter of two piece ammunition. The retainment by initially the Soviet forces, and later that of the Russian Federation I think has primarily to do with the introduction and retainment of the automatic loading system in its prevalent tank designs. The automatic loading system cancels out the time- and handling-penalties that 4-man tank crews with loaders (however well-trained) have in designs such as the American M60- and M1, the German Leopard 1 and 2, and French AMX30 and 40 series. These NATO-tanks all switched to 1-piece, combustible cartridge ammunition from the 1980s/early 1990s onwards.

With this in mind we can observe 2 main points, and another 2 tangential arguments for the retaining of 2 piece ammunition; 1) it is cheaper and less complicated to manufacture than combustible cartridge ammunition, 2) there was no direct necessity for its introduction, as the automatic loading system was easily able to configure the handling of two separate elements of ammunition. Added to that 3) loading systems from the mid-1960s onward also allowed for the introduction of 1-piece guided missile ammunition in a 125mm smoothbore gun (the 2A46 if I'm not mistaken) to serve in the advance anti-tank role, and 4) political forces in the higher rungs of planning- and design facilities in the Soviet Union were deeply divided or indifferent towards developing 'modern' 1-part regular ammunition for newly introduced tank designs.

So the majority of this information comes from Jane's Armour and Artillery from the mid-1980s, Chris Bishop's Encyclopedia of Tanks and Armoured Fighting Vehicles, and some quick (an possibly terrible) wikipedia searches. As usual, correct me where I am mistaken! Hopefully this helps you look in the right direction!

EDIT: Removed AMX 50 series; AMX 50 was a predecessor to the 30-series and never saw mass production or introduction into service. I intended it to mean AMX 40 series, but also realised there that this design never made it past the prototyping stage.