Looking at Alexandria Ocasio-Cortez's race, Eric Wimer has a write-up of the seemingly insurmountable benefits that an incumbent has against primary challengers.

Wimer focuses on tangible, hard power, but another factor is just inertia. Min and i recently volunteered for a progressive candidate in a local race on election day. I thought we'd be helping with GOTV efforts, making sure that identified supporters went to the polls. Instead we were asked to do last minute campaigning outside the polling stations (at least 100 feet away, of course!). We were willing to do whatever, but privately i thought that this was a dumb use of our time. Who hasn't already made up their mind by the time they went to vote? It turns out that the answer was... nearly everyone. Most people we talked to didn't really understand that the election was a primary - they just wanted to vote against the Republicans - or they hadn't thought about the local race at all. And everyone that we talked to was in agreement that the candidate we were supporting was on the right side of the issues (mainly, the tax abatements given to new buildings in the area, which not only reduce the amount of revenue going to the community, but take ALL of that money away from the school system). So we converted quite a few voters and it turned out to feel worth it. But of course we were only at one polling station for a part of the day. Our candidate was part of a group that was well organized and had a lot of active support and was on the right (and seemingly popular) side of the issues. But they didn't win against what felt to me like the zombie-like horde of people that just went out to automatically vote down the party line column. The structural advantages that an incumbent has are just huge.