I suspect it’s due to unfunded public pension obligations.
Almost all states havep the same unfunded pension problem.
I read something awhile back that looked at all 50 states and their pension system. I think that of the 50 states there are only 4 or 5 that are funded in excess of 40% (not positive about this number but it's close).
Anyway, the point I was trying to make is that Illinois catches all the chit for poor financial operations and is constantly referred to as “worst in the nation” etc. and as shown in this article, California's debt is better than 10 times that of Illinois.