If you analyze the NotPetya attack, it differs from other ransomware attempts in two respects. First, it was specifically targetting Ukraine. Second, the attackers didn't actually take any money but rendered all systems defunct. If you are a criminal, you aim to make money, right? Why give up on that possibility? It makes no sense.
So, even if in the infosec world you can never say never, but just as Stuxnet is generally attributed to Israel/USA, in the same way NotPetya is attributed to Russia, even though none of these countries will ever admit they actually did it.