Why is the nominating process so complicated?
One of the basic principles of our democracy is the separation of powers, among the branches and levels of government. The writers of the Constitution were suspicious of concentrations of power, so they created a variety of geographic levels of responsibility, with different officials elected by different groups of voters. Power is divided among them. Each elected office has a separate nomination process because it is decided by a different geographic area: a State House or Senate District, a County, a Congressional District, a Judicial District, the whole State, or the whole country. Here’s a map of the process.