Minor political parties have such little success competing in US elections. One reason has to do with the decision of the founders of the country to adopt a Presidential rather than parliamentary system. Add to that the size and complexity of the country: Only very large and well- funded parties can organize the country-wide campaign needed to compete for the Presidency. Once such parties came into existence and developed formal structures for selecting candidates and conducting campaigns, new entrants found it hard to compete, even more so today when vast sums are required to run campaigns for the Presidency and also for members of the Senate and the House of Representatives.
A second reason is the absence of a system of representation in the legislative branch in proportion to the national vote or the vote in each state. Races in the states for election to the two national legislative branches are run with only one exception of which I am aware on a first-past-the-post basis. Each state divides itself into Congressional Districts and in each district there is only one winner. Therefore, even if a new party were to win 20 percent of the vote in the state as a whole but did not win a majority in any one of the Congressional Districts, it would not succeed in electing a single member of the national House of Representatives.
A third reason is that historically each of the two established parties have functioned as big tents within which groups with very different policy ideas co-existed. In other words, each of the major parties has been a coalition and one or the other has been able to absorb new interest or identity groups as they have emerged from changes in society and political economy. What unites the coalition is (a) the Presidential competition: By being part of the coalition that elects the President groups within the party have a chance to influence national policy and to be selected to positions in the Government and (b) the chance to get one of their members chosen at some point as the Party’s Presidential candidate.
The founding fathers of the United States were from the upper and upper-middle classes and had a certain distrust of people who were not well educated and lacked property. Therefore they distrusted simple majority rule. They were also influenced by the size and diversity of the country even though at that point it had only 13 states scattered along the Atlantic Coast. Those states had begun as colonies founded and populated by people considerably different in culture and political and religious beliefs. After the War of Independence from Great Britain, each colony became in effect an independent state. They varied from each other in size, economic interests and population. In order to convince all of them to surrender some of their authority to a central government, it was necessary to guarantee to each of them a certain degree of political power irrespective of the size of their population.
The Electoral College is an institutional expression of the concerns of the founders. They envisioned the electorates of each state electing members of this body who would in turn vote for the candidate for President each of them preferred. There were no well-developed political parties yet. So the drafters of the national Constitution imagined the voters in each state selecting the better educated and wealthier persons in the state to serve in the Electoral College and these persons would exercise their individual judgments about whom to select for President.
In addition, to satisfy the concern of the states with smaller populations, it was agreed that they would have at least a certain number of Electoral College members even if their proportion of the national population were to diminish greatly. Indeed from the beginning states with smaller populations, which also tended to be more rural states, had a number of electors out of proportion to their populations. For the same reasons, every state was given two Senators and has continued to have two Senators to elect to the national Senate even as the population asymmetries among states have vastly increased. So today, a state like Montana with a population under one million has the same number of Senators as California with a population over 30 million.
The overrepresentation in the electoral college of states with small populations is not as great, but it exists to some degree and that, plus the tradition of first-past-the-post and the decision of most states to require that persons elected to the electoral college with a party identification cast their vote for the person nominated by their party to run for the Presidency has resulted in recent decades in persons being elected President while losing in the popular vote count. Suppose Trump won the popular vote in Texas by only one vote more than Biden; he would still get every one of the Electoral College votes from Texas because candidates are selected today for their party identification not for their individual qualifications.