Originally they were held in a way to expedite the process instead of bringing all the ballots to one location for counting. This quirk later became enshrined because of conservatism.

For political parties and primaries, there was no model at all in the foundation of the country, and there was actually an inclination amongst the founders against having political parties at all. Moreover, the 10th Amendment pretty clearly puts the individual states in control of party formation rather than the federal government. Hence why you'll have a D or R party in each state that's relatively independent.

The FPTP method of elections ensures that everything will drift towards 2 opposing parties.

So we have the Electoral College, and we have political parties with make-it-up-as-you-go structures that end up being rather byzantine. What parallels this is increasing pressures to lengthen the election season. A longer election season means that small parties and poorer candidates are effectively shut out, and moneyed interests will naturally favor this: spending more money on elections is worth it if it ensures candidates friendly to them are winning all the time.