Narrative has been consistent with any aggressive invader - find any justification and play it up. They continue to hold this line because they've ended up in a protracted conflict that they were not expecting. Took Crimea with little serious international grief. Been fighting in Donbas & Luhansk since 2014 without any serious international opposition/aggravation so consolidating that area would have been far less controversial or provocative.
Maybe could have simply directed all their efforts in Feb 2022 into Donbas & Luhansk. Would have been simpler and presented the "West" with a fait accompli. Let's not kid ourselves - if they had reached Kiev in that first week they would have taken the whole country for themselves.