The issue is that the politicians benefitting from First Past The Post voting would need to be the ones to enact preferred voting. They benefit from FPTP because it locks voters into the two party “any third party vote is wasted” system. And they have zero reason to change that, because they would be legislating themselves out of office. The two big parties would suffer massive losses if people were able to vote for who they actually wanted, so those two parties have a vested interest in preserving FPTP voting systems.