Primaries are not an election. Get that out of your head.
The Democratic and Republican parties have the right to run whatever candidate it wants without any input from voters. There's NO provision for Primaries or political parties in the Constitution. It used to be Congressmen just nominated whoever they preferred from their respective parties to run for President.
Both parties are well within their right to just TELL us who the candidate is going to be without soliciting our input. That's exactly what the Republicans did last election. They didn't even have primaries, Bush was the appointed nominee.
The sooner people understand that, the better off we'll be. If you don't like the system and want to change it, it usually helps to understand what the facts are.
To address your point about caucuses
In order to find the best possible nominee, the Democratic Party consults a combination of ordinary normally lazy voters who get their news from the MSM AND Activists. Caucuses are their way of telling how the hard-working activists who've been paying attention to the issues and put their energy where their vote is are willing to work hard for.
They have Primary elections and caucuses so they can get a good picture before they invest for the kill.
I'm sure you can fill in all the dots to understand why this system makes the MOST sense for the party.