Actually, for the last couple of decades, the primary process in both parties has become very open. There isn't even a requirement to be some sort of formal member of a political party, you only have to be registered (and in some states not even that).
There's a widespread conception that candidates are picked via meetings in smoke filled back rooms - that used to be true, but it isn't true (certainly not in the same way) anymore. Read up on the stories of various "insurgent" candidates in both parties over the last 20 years - the common element is organizing and campaigning so that primary voters actually check the box for you.
Didn't the democratic party just outright state that they could choose who they wanted without any input from the people and had no obligation to run a fair primary election? They went to court to defend their right to keep choosing candidates picked via meetings in smoke filled back rooms
Or was there an implied /s there?