The voting system used has lead to a democracy of only two parties (Duverger's Law). There's a very strong us-versus-them mentality comparing the Democrats and Republicans. Because of this, the interesting political debates don't really happen at the party level but at the candidate level - so it matters a lot which person is nominated, because if they win it's their ideas and policies that they will try to drag through Congress.

If you want to know why it is in the news every. single. day, Ryan Holiday explains in his book that news outlets like Politico figured out that they can start coverage of the race years in advance. They do this simply by discussing and speculating on potential candidates and ramping up the coverage from there on. This election has been nuts (at least from my perspective) in no small part because of all the media attention generated around it.