At the moment because they are not well enough developed beyond doing very simple equations . The term quantum supremacy as used by google is misleading as it only occurs over normal computers when applied to a very simple problem . There is considerable doubt as to whether Quantum computers could ever crack SHA -256 . It is theorised to be quantum resistant . The most efficent theoretical implementation of a quantum computer to detect a SHA -256 collison is less efficent than traditional methods of cracking and still requires knowledge of the public key. In the meantime developers are working on maintaining computer resistant coding
further reading here
https://royalsocietypublishing.org/doi/pdf/10.1098/rsos.180410
https://www.forbes.com/sites/rogerhuang/2020/12/21/heres-why-quantum-computing-will-not-break-cryptocurrencies/?sh=65dd3a81167b
