Maybe because that rationale shifts depending on what day of the week it is, whether it's to save Russkies, re-instate the Soviet Union because letting Ukraine go was a mistake, or "Bio-Labs" announced months into the invasion--with Russkies pumping out documents that either do not say what they claim they do, or which are utterly fraudulent, instead of, you know, inviting scientists to all the biolabs they've supposedly destroyed or captured since this all began to prove their existence.
Putin’s rationale was stated early on — it’s not a matter of surmising given whatever the daily/weekly/monthly pronouncements indicate.