[mreome]

I think it has a lot to do with the two-party system. Many (most?) congressional districts are essentially locked to one party. Unless the incumbent did something egregious enough for their supporters to turn on them, why would their party not support them as their candidate? If they won by decent margin last time, they'll probably win again unless something major changed. Why make all the necessary investments to support a new candidate when it's cheaper and easier to support the incumbent? They would much rather put their money and time into the districts that might flip.