They haven't been able to modernize due to budget.
This document from 2017 was written by the US military and analyzes and points out weaknesses in the Russian BTG (Battalion Tactical Group) doctrine and has some excellent points that make a lot of what is currently occurring fall into place.
https://www.benning.army.mil/armor/earmor/content/issues/201...
>Current Russian state net revenues from energy cannot fund the same modernization period as before.
Basically they can't modernize.
It then goes into the current Russian BTG doctrine and how it leaves armored groups exposed due to a lack of supporting infantry and air.
>The BTG’s may be vulnerable to raids, counterattacks and other surprise movements because reliance on analog C2 limits subordinate units’ ability to understand and react to changes of circumstance.
>A BTG had the entire brigade’s support and enabling resources, but it had only one mechanized-infantry
battalion
>According to Russian Army manuals, in the field as many as 50 percent
of infantry soldiers can be required for local security and routine administrative tasks. This leaves relatively few infantrymen available for mounted squads. Squads are usually organized ad hoc and are less than fully manned,which makes them less effective and less independent. For opponents, it also means that it requires fewer casualties to neutralize the Russian squads.
>The lack of infantry causes BTG commanders to prefer to isolate urban infantry strongpoints for prolonged sieges instead of assaulting
The document also goes into supply issues that BTGs will likely face.
Basically the current Russian doctrine is ridiculously ineffective. It involves lots of independent battalions of almost pure armor. Which get destroyed consistently due to a lack of supporting infantry. To the point where it's quite possible they'll lose this war. The Russians thought it was effective in Chechnya and Syria and the 2014 war with Ukraine since they simply didn't face significant resistance. All of the above weaknesses are now very apparent however.
One last thing
>The current military and political leaders are the same leaders who introduced the BTG structure; their reputations
and careers are closely tied to its success. Instead of moderating BTG rollout and keeping a portion of the Russian
Army in a divisional structure to train for high-intensity CAM, Russia’s leaders are accelerating the rate that units
convert into BTGs.