With regard to the CA bar specifically, it has one of the lowest pass rates primarily because it permits anyone to sit for the bar irrespective of whether or not they graduated from an ABA accredited law school. Most other states, by contrast, only allow ABA grads to sit for their respective bar exams. When you compare ABA stats to ABA stats, it becomes apparent that California is not the "hardest bar exam in the country" that most think it is.
Take look at the statistics available on Calbar's site. Non-ABA grads made up a whopping 6% of first time test takers last February. If every single non-ABA grad failed, the effect would only be 6%.
Fully accredited ABA schools from out of state often have pass rates in CA that are 20-30%, even though their in state pass rates are 80-90%. The fact that CA has a tougher than usual bar exam is evidenced not by the handful of non-ABA grads who fail, but by the thousands and thousands of fully accredited ABA grads who fail.