Russian military logistics are very reliant on railways. Ukrainian forces destroyed all the rail links between the two countries immediately the war started.
Russia doesn’t have enough trucks. For their number of trucks and the size of their army, the army can’t operate effectively more than 90 miles from the supply dumps at their railheads — when the trucks can run back and forth unimpeded.
Fuel trucks, especially, are targeted by the Ukrainians — they are soft targets, and without fuel the Russian tanks are useless.
A large percentage of truck transport was dedicated to rocket resupply, not fuel and food. Russia only had 3-5 days of supplies when they invaded. When those ran out, they were left floundering.
Russian forces are relying on analog, unsecured radio communications rather than scrambled digital channels, and Ukrainian forces are listening in and jamming.