If you want a simple answer: Cost of living, primarily.
California has a wide array of problems but what has to be understood is the state's size is massive compared to most other US states and it has a wide array of geographical regions which are, again, each host to a variety of different problems. One could almost ask "What is wrong with the United States?" and receive similar answers. Where to even begin?
To put this in perspective - Great Britain has 63 million people. California has 40 million. Again we have all the same problems that entire countries have.
Specific issues that the state faces? Homelessness - almost 50,000 homeless in LA county alone. There is a serious water crisis, that is affecting the entire state, but particularly in Southern California where there is a lot of agriculture in areas that probably shouldn't be farms. There are regions of the state which will run out of water unless substantive policy changes are made.
I think there is a lot of dissatisfaction with the state government, despite being a single party supermajority, for decades. Why has the liberal paradise not been realized? It just seems "nothing" can get done. Incompetence, corruption, etc. Confirmed unemployment fraud during the Pandemic is confirmed at over $30 billion dollars. California creates dozens and dozens of new laws every single year but I'm not so sure they're actually making the state better for Californians.
There are a lot of economic issues.
California is probably great if you're wealthy enough to live there.