People like to imagine that the Parties are these kind of quasi-public entities that exist to serve the American people. They’re not. They’re private entities that are vehicles for the acquisition of power across local, State and Federal offices that are in a perpetual term-based competition with each other. We tolerate their campaigns because their battlefield is the ballot box instead of actual fields, but nominee selection still happens at conventions run by the parties themselves.

The Primary system you are familiar with is one that the Republicans and Democrats chose for themselves to inform their overall decisions on who to put forward as the candidate flying under their banner and has only been around since 1972.