Wars are won (and lost) as a result of supply chain logistics.

One of the most important elements of the war machine is a well-fed and well-equipped soldier. As such, most armies have dedicated logistics divisions to ensure supplies, weapons, and machinery get to where they are needed on time.

Consequently, supply chain interdiction or disruption is a powerful weapon of war. To interfere with your enemy's logistical operations is to reduce the overall effectiveness of their combatants and thus their strategy.

Modern-day Russia was not prepared for this war to last as long as it has, their commanding officers have little regard for their soldiers, and they've historically had poor logistics to begin with.

This makes it easy to disrupt their supply chains, if they were even set up in the first place. Add this to tribalist infighting and a general lack of cohesion and you end up with people going hungry.