Because the left wing position is that a democratically elected government should be deciding how money is distributed, while the right thinks government should be stripped of this power. Think of blockchain as the privatization of money supply.
I don't think that's necessarily the case. With that criteria, the concept of a society without money at all would also be a 'non left wing' position, and I don't think that's the case. Decentralised currency doesn't strip the government from the ability to redistribute money --- it can still do so via taxes, even without control of the printing machine.
Hell, the right implementation of the decentralisation of currency (obviously not what we have right now with bitcoin) could be more equitable than what we have right now with centralised currency.
Can’t really talk beyond Europe viewpoint, so might be different elsewhere, but here in the UK the left often talks of removing Westminster central control and empowering smaller local politics. I’m left wing/labour/union/coop worker and work in crypto, same with my colleagues. I really disagree with the projection that it is right wing ultracapitalist. It’s one of the best tools I’ve seen for cooperatives and community organisations