In Western Europe for example, many legislatures have a party-list system and are structured with proportionality, but in the US supporters for a change of system are almost entirely nonpartisan. The reform movement exists with interest groups and third parties looking to gain representation.
Why is it that neither the Republicans nor Democrats are interested in proportional representation or other more representative systems?
Duverger's Law indicates that plurality (first past the post) systems devolve into two parties per seat, while proportional systems support many parties. Because the United States has a national presidential election, this essentially forces the US into a two party system.
If the US switched to a proportional system, it would make it easier for third parties to get more votes and seats. Those seats would have to come from the Democrats and Republicans because the two parties have almost all the seats (currently 98% of the Senate and 100% of the House). Since both parties would lose seats, both have a vested interest in avoiding a proportional system.
Individual members of either party might be more open to the idea. The ones who are the most interested will often be those most likely to join a new party.
In the 2017-2018 Congress, the Democrats control 47% of the Senate and 45% of the House of Representatives while the Republicans have 51% and 55% respectively. Contrast that with Germany, where the largest party is 32.9%. Other proportional systems have similar results. No one party controls a majority of the seats
Out of power, the Democrats have more incentive to shake things up, but they also have the broader coalition. They are in many ways more vulnerable to a proportional system than the Republicans. In Germany, the second largest party is only 20.5%. This is why they tend to concentrate on reforms that increase their influence relative to Republicans (e.g. the Fair Representation Act or efficiency gap analysis) rather than reforms that reward proportionality.
Now, sometimes the larger parties do better than at other times. Before the latest election, the two largest parties in Germany controlled 41.5% and 25.7% of the seats. And of course the Democrats may find that they would still be the largest party in the coalition.
Meanwhile, the Republicans are a majority in both chambers of Congress and control either the governorship or at least one chamber of the legislature in most states. They might be the largest minority party under proportional representation, but they would still go from the majority they have now to being smaller than they were when last in the minority.
There are countervailing advantages. For example, if congressional Republicans and Donald Trump were in different parties, they would be less beholden to the president. And they would have more options in coalition building. But apparently they don't value that as much as they do the current system, where they are at worst second.
This question assumes that in the US the minority and special interest parties are frozen out of the political process by the 2 major parties.
However what actually happens is that the minority parties that form (at least the ones that have formed in the last 80 years) form based on specific issues. Women's rights, workers rights, black's rights, gun rights, whatever.
When this happens if one of these parties happens to strike a nerve and start gaining traction among the voters, and gain membership, then one of the 2 majority parties simply co-opts the specific issue for themselves, and most of the voters of the minority party go to the majority party that co-opted the issue.
The single-issue party has, at this point, completed it's mission - they have successfully prioritized their issue to the point that it has been taken up by a major party with the muscle to get it implemented. They don't need to elect members to get their agenda implemented now. They have in fact NOT been frozen out of the system - they have been successful in changing the system.
But the leaders of the successful minority party don't see it this way. They figured they were going to get some power for themselves - and now they won't. Their issue is being worked on but they wanted more than just that, they wanted to wield power without sharing it and hoped to use their issue to leverage themselves into power. Needless to say they do not want to publicly admit this which is why all of them claim that we need proportional representation and an end to the evil 2 party system.
If one of these days in the US a 3rd party formed that was not single-issue and had a comprehensive platform that was attractive then we might see 3 major parties. For example a 3rd party that was socially liberal - supported women's rights, abortion, opposed religion in schools, strong environmental laws, strong limits on corporations, etc. etc. - yet fiscally conservative - supported government spending reductions and a balanced budget - might just do it. That would end all the talk of the 2 party system being against proportional representation. But, don't hold your breath waiting for this because to start a minority party takes someone with tremendous energy that can only come from someone who is either doing it because they truly want to better everyone - like Martin Luther King - or someone with a tremendous chip on their shoulder - like Ross Perot. And the ones with chips on their shoulder have the chips due to single-issues they are angry over and there are far more of them than the other kind.
To put it simply, it's not in their best interests. First-past-the-post favors a two-party system, as it forces all other political parties to either ally with a side or sit out of government. Likewise, the debate apparatus is owned by these two parties, allowing them to shut out those that don't conform to 'Democrat' or 'Republican'.
This has created a very effective adversarial/sports-team sort of setup of heart over head. It also has the added benefit of getting people to vote against their own self-interest if you focus on another issue that you and the voter agree on, as the other side is worse.
The Democrats and Republicans have way too many skeletons in their collective closet to survive as parties (in their current forms, anyways) in a ranked choice setup. Because of the adversarial setup, a ranked choice system would have voters spending their second, third, etc rank on anybody but the major party they hate. Taken collectively, some new, probably moderate party would gain power by being everyone's second choice.
s/nonpartisan/third party/g