Politiciians will be responsible for considering the implications and the details of how to enact it - whether the laws protect against undue influence, manipulation, exploitation, how feasible it will be to arrange oversight and who will be responsible. They ultimately have to make the decision based on whether what is proposed meets what the general public want -and they would be blamed if the passed a law that failed these basic safeguards, regardless of what the public want.
Give that there are huge problems with how the Mental Capacity Act is followed and overseen, frequent issues with vulnerable adults being given inappropriate DNRs without following the proper procedure (particularly in the case of people with LD or cognitive impairments), just to name a few examples, politicians may have very reasonable concerns.