Changing the voting method is much easier than changing to proportional representation, or any other means of avoiding a single winner. The former is something that's under control of the states. I...think the latter would require an amendment to the Constitution for Congress, and changing the nature of the Presidency definitely would.

Even just allowing people to provide more than one vote means that people can support third-party candidates without that vote effectively robbing their preferred major-party candidate of a vote. (eg, if you're broadly left-wing, and like the Green Party, you can rank their candidate first, then the Democratic candidate second—and then if the Green Party candidate doesn't win, your vote counts for the Democrat) That's a big, big change.

> I...think the latter would require an amendment to the Constitution for Congress

I don’t think it does [1].

[1] https://www.law.cornell.edu/constitution/articlei#section4