1. The counties downstream hold a majority Republican population.
2. (More importantly) Federal and State funds were shoveled into the coffers of Granite Construction, heavy contributors to Democratic candidates in California, to construct a completely useless, albeit massively expensive, spillway project at Folsom Dam. The spillway repair at Oroville was just too diminimus to waste Granite's time.
But, if you just try to prove this unwarranted accusation, expect to find yourself as a bunkmate with Jimmy Hoffa.