It's important to remember that first and foremost the various party organizations are incumbency protection rackets. While taking and retaking Congress as a whole is of course a goal, a more important goal is keeping as many of their own jobs as possible. These goals obviously aren't entirely at odds with each other, but nor do they necessarily align perfectly. In a year when incumbents are feeling nice and safe there's plenty of money to throw around to challenge incumbents and try to win open seats. But in a year when the incumbents start fearing for the jobs the money is going to start flowing to them.
And, yes, yet another reason why it's not so crazy to run candidates everywhere. It can really help to suck all of the money out of the system.