A lot of misinformation has been spread over the year about California. Everyone isn't leaving California and people/businesses with financial ties to the state must continue to pay taxes even if they leave.
The federal government discontinued funding for affordable housing in the 70s throughout the country, and automation and outsourcing jobs caused a lot of job losses. Low wages, the financial sector's housing inflation, and federal funding for public programs also contribute to the issues, and people still haven't rebounded from the financial crisis in 2008. This isn't to say California has had perfect leadership, but there's a lot going on.
California is the fifth-largest economy in the world and the state has the highest corporate tax rate, but corporate owners like Elon Musk cut "expenses" so they move headquarters to states with lower corporate tax rates.
California's population
