Why? Russia, and the USSR through the entirity of the Cold War, delivered reliably all the gas ordered. If you have a stable relationship with suppliers stretching decades and through all kinds of conflicts you have zero incentive to change. The infrastructure is there. And being in business with your "enemy" keeps channels open and results in intet dependencies.
Putin, in a bout of insanity, through all that out of the window. Once it was clear that's gonna happen, and no that was NOT clear a year ago, it was too late to switch (infrastructure).