Climate change is making the drought worse, and California's water system was designed for a climate that doesn't exist anymore.
Even before the current drought in the west, the way Colorado River water is distributed is based on data taken from years of above-average precipitation.
In other words, they based water distribution on a glass-is-overflowing model.
Combine that with warmer temperatures and a Sierra snowpack that disappears in the blink of an eye, and we're entering uncharted megadrought territory.