I don't think poor result have anything to do with the applicable law on the individual bar exam, but it has a lot to do with the stress of adjusting to a new environment. When I was taking the California Bar I was freaking out, but I went to law school in San Francisco and had my friends, girlfriend, apartment, etc in San Francisco and it was somewhat comforting to have a support structure during that time.
Had I moved across the Country to New York and not known a soul, rushed into some random apartment, and knew nothing the stress of all those changes would have been very difficult to deal with and the results may have come out very differently. This is something I don't think many law students take into consideration when studying for a bar or choosing a law school.
Yes, I agree that it's definitely a factor but to what degree? The pass rates for many out of state schools in CA is really low, like 25-50% even for first tier schools. A lot of those students are actually from CA and are returning home after law school, so they do have a support network. But as you say, others don't.
I just wonder if CA law schools prepare students for the difficulty level they will face on the CA bar exam, whereas other states prepare students for their own level of difficulty? It would interesting to see the rates at which CA students pass/fail other states' bar exams. I've never taken another state's bar, but I know people who have. With a few exceptions (NY and TX), most of the people I've to felt that CA's bar was more difficult, and in some cases significantly so.