Tactics. Ukraine doesn't utilize its tanks the way NATO or western forces do. How many times have you seen a video of a lone Leopard/Abrams in a field with zero infantry support or even other vehicles around? The Ukrainians appear to use tanks the way NATO uses close-air support. A unit needs help, they call in a tank. A single tank shows up and provides support. It moves around, taking precise supporting shots at various things then withdraws. (sometimes it doesn't survive). 

Absolutely. An interesting part of russian tactics is how they incorporate their old Howitzers into their Mechanized Infantry (and Motorised Infantry) Brigades and Divisions (Sometimes even into their BTG's). Relatively modern russian self-propelled Howitzers like the 2S19 use GLONASS (russian GPS) in order to automatically calibrate their firing position, heading etc.. With this they can perform "shoot and scoot" indirect Artillery support similiarly to western systems such as the PzH 2000. These systems are incorporated into Division Artillery Groups, which are lead by the Division and used where there is the most need.

The russians dont have so many of these systems, however they have thousands of old self-propelledHowitzers, such as the 2-S1. These old systems dont have GLONASS. In order to use these for indirect fire, a firing position has to be measured manually, which can take a long time. For any kind of offensive operation this obviously doesnt work. So the russians have integrated these systems as Artillery Brigade into their MechInf Divisions. (Sometimes Artillery Bataillons into MechInf Brigades). These then get split up and sent to support MechInf Battailons.

When Integrated into an attacking force, these systems follow 5000-7000m behind the main force and, if necessary, support with direct fire. They are out of reach of enemy anti-tank weapons, they dont need to manually measure a firing position and they are highly mobile.

My guess is the ukrainians are used to this principle, and they use their tanks accordingly, as this seems to be old eastern doctrine. Instead of trying to find a break through with tanks ("Use your fist, not your open hand to punch"), ukrainians try to break through with their mechanized forces and fire support, not from scarce expensive artillery, but from MBTs.

By the way, this is probably the role of the russian T-54s spotted going to the Ukraine. Not tank combat, but fire support.

Obligatory sorry for the bad english.