As I understand it as a foreigner, some/many Democratic primaries are decided by registered Democrats voting for their preferred candidate. While the vote at the general election might see most voters already decided, the candidates that get onto the general election ballot would be influenced by those that register with the party. Over time - and without forgetting the advantages of incumbency - this could skew the type of candidate that appears on the ballot with a D next to their name, and lead to candidates that even consistent Democrat voters might reject.
Hypothetically that is what is supposed to happen, but in action, politics has become so polarized, that swing electorates are almost non-existent.
At this point elections are won based on whether or not subsegments of the population can be rallied to show up or not show up to vote in elections.
This is becuase in most elections, most voters simply do not follow the news, and if they do it tends to be a quick video or a listicle.
Thus, the voters that can swing an election are those that are part of organized voting blocs (eg. A specific union or a local PTA), and cultivating those local groups and ties matters more.
Mainstream news is boring, repetitive, uninformative, and mainly representative of the extremes of politics. This type of programming is not attractive to most people, only shallow political junkies consume it voraciously. Most people care about healthcare, housing, education, and community. The value proposition for most people to watch the news is not very attractive.
