Because there are only 2 parties. It would be undemocratic to restrict who is allowed to run in them. More practically, the parties are not homogeneous, and the

Because there are only 2 parties. It would be undemocratic to restrict who is allowed to run in them. More practically, the parties are not homogeneous, and there will always be factions that align more closely to groups outside the party. The most relevent legel precedent I am aware of is Smith v. Allwright (1923), which ruled it was unconstitutional for a political party (the Democrats) to restrict primary voting to whites. >When, as here, primaries become a part of the machinery for choosing officials, state and federal, the same tests to determine the character of discrimination or abridgment should be applied to the primary as are applied to the general election > We think that this statutory system for the selection of party nominees for inclusion on the general election ballot makes the party which is required to follow these legislative directions an agency of the state in so far as it determines the participants in a primary election. The party takes its character as a state agency from the duties imposed upon it by state statutes; the duties do not become matters of private law because they are performed by a political party. Granted, in this case, the court was looking at racial discrimination by the parties, which introduces direct constitutional concerns, and so has a lower bar to warrent governmental interference. However, the principle that "private" political parties are not actually private is fairly engrained in US politics (although the exact legal boundaries are still unclear). https://supreme.justia.com/cases/federal/us/321/649/#tab-opinion-1938079 https://supreme.justia.com/cases/federal/us/321/649/#tab-opi...