Think of the money it would take to campaign nationally for a single day primary. I like the gauntlet that the candidates have to run. Small media markets like Iowa and NH are a good way to cull the weak and yet a way for those less known to step forward.
So why even have primaries? Madison, Wisconsin is a bastion of demodummies but the rest of the state is usually Republican. Don’t judge the entire state by Madison, WI.
The county I live in has approximately 23,000 population (yes we are small) and is mostly conservative but voters in the county seat of 8100 is akin to Madison, “demodummies” so this county is then crowned as “democrat”? Those 8100 people DO NOT represent the entire country. Same goes for primaries. Does a small sample of the population tell you what the entire voting population wants?