Donbass is the most resource rich part of Ukraine. It's a fact.
This whole thing actually makes more sense if you start looking at it through a prism of a fight over resources.

Western corporations were set to exploit those resources before the 2014 crisis.

Ukraine crisis unsettles Shell and Chevron

https://www.ft.com/content/82ea0fff-a905-3cb9-921b-9031d507638e

According to some sources, there are trillions worth of dollars of untapped resources in Eastern Ukraine. Including all sorts of things. Oil, gas, lithium.