There are plenty of such parties. They are just not electorally successful on a national scale. They may be moderately influential on a state level through the use of fusion voting. Fusion voting is where multiple parties can stand the same candidate in an election.

Most places in the United States use a “first-past-the-post” system. In this system, voters select one candidate and the candidate with the most votes wins. This system sounds fair on the surface but in reality, game theory dictates that the only stable configuration of political parties in such a system is a two-party system. In any other configuration, the individual actors will always find it more advantageous to join one of the two parties. The reason for this also explains why it’s hard for smaller parties to win under a first-past-the-post system.

Suppose there are two existing political parties: Party A and Party B. Voters prefer Party A by a margin of 55-45, so Party A wins reliably in elections. Suppose we replay the same elections but with three parties. Party C holds similar views to Party A but is more extreme while Party A is more centrist. If everyone votes for their favourite candidate, then we will probably end up with a vote distribution where Party A wins 40% of the vote, Party B wins 45% of the vote, and Party C wins 15% of the vote. What has essentially happened here is that Party C siphoned votes away from Party A, causing Party B to win despite the fact that voters’ preferences haven’t changed. Voters know this and so even those who like the Party C candidate the most will vote for the Party A candidate (who shares at least some of their views) in order to stop Party B from winning.

This is why progressives forming their own political party is a losing idea in the United States. It will split the left-wing vote and hand elections to the Republican Party. Instead, what they do is compete in the Democratic Party’s primary elections. In the US, who a party chooses to stand in a particular election is determined by means of a primary election. However, progressives often struggle to win intra-party primary elections because most members of the Democratic Party are moderate. The distribution of political leanings is shaped like a bell curve, and thus progressives like Bernie Sanders are simply outnumbered by moderates like Joe Biden. Moderates often have the numbers to sideline progressives in primary elections, and thus it is much more difficult for progressives to get elected since they need to run under the Democratic Party banner to stand any chance of winning.