America is, fundamentally, a center-right country. This is obvious when you compare the two American parties to their more numerous counterparts in other countries. In most other developed democratic nations even the party considered by their standards to be center-right is to the left of the Democratic Party on most issues.
In addition to simply representing the actual mindset of most Americans, a large part of the reason for this is state of affairs is also structural. Whether how American politics is structured caused Americans to become more conservative over time or America’s conservativism led to its structural deficiencies, I’ll leave it to you to decide. There seems to be some kind of self-reinforcing feedback loop at work.
Regardless, unlike other democracies the United States has only two major political parties, one of which must represent under its “big tent” everyone from Joe Manchin to AOC and Bernie Sanders. Were the United States a parliamentary democracy it would likely have several parties represented in Congress, each representing a smaller segment of a population that currently has only two real choices on election day. Given the present reality, in many districts and some states remaining closer to the middle is the best strategy for candidates trying to get over (or as close as possible to) 50% of the vote. Because Democrats are the party with the most diverse membership, finding a way to be as appealing as possible to as many as possible is also rewarded in their primaries to a far greater degree than it is in the less culturally and ideologically diverse GOP. The Democratic Party’s diversity of constituencies also means its message is often more muddled than the far more homogenous Republican Party.
While most countries have electorates that are to the left of the US electorate, even in these other liberal multi-party nations the most progressive/liberal parties don’t often find themselves in power unless they are part of a coalition government. Even in winner take all or so-called “first past the post” parliamentary systems, victory typically means getting a plurality of the vote as opposed to a majority. Up here in Canada, it’s been a while since either the Conservatives or Liberals formed a majority government based on more than 50% of the total vote. That’s because these two major parties (the only two to ever form government in Canada) are dividing the vote with the social democratic NDP, the Greens, and in Quebec the Bloc Quebecois. It’s worth noting that all those parties are to various degrees ideologically left of the Liberal Party. That even though Canada’s Conservative Party has the right all to themselves and the left is splitting its vote up three or four ways and the Conservatives still only form government about as often as the Liberals is a pretty good indication of just how much more liberal even the average Canadian is when compared to the average American. Even the Conservatives up here support universal healthcare and have generally steered clear of culturally divisive issues like a woman’s right to choose or gay marriage.
So, to make a long story short, if you want to make America more progressive you might want to start with structural reforms that will incentivize greater diversification of representation. Until that happens, it’s hard to imagine things changing there any time soon barring some sort of crisis like the Great Depression forcing it to dramatically change course.