I have no real idea, only suppositions :

(I'm not treating Ukraine as the enemy, this is a full supposition).

The real hard part is costs and time. If the soldiers pushed the front lines, and there is no risk of enemy attacks behind the line, there can be multiple things in the way :

The terrain may not always be good for fast travel. With difficult terrains or mud and water.

And because front lines aren't perfect, there is always the risk of enemy attack behind, destroying supplies.

Now why exactly Russia did not send supplies to their soldiers, no real idea. They only know what they chose to do.

Ukraine is huge, giant. Going up to the front lines takes time and money. And well I very much guess that Russia just sent soldiers without care, maybe even now they doing that. They underestimated Ukrakne's defenses and the support they get from other countries.