Not saying I disagree, because I think the bar association is a cartel and everyone should be "allowed" to practice law. However, the reason the CA bar is "notoriously hard" is precisely because they allow anyone to sit for it. If lots of unqualified people take an exam and fail, it causes the passage rate to drop and makes the exam appear "harder."
Not totally true, it's legitimately harder. First time taker pass rate for even people going to tier 1 law schools is significantly lower than many other jurisdictions. The addition of allowing an expanded group to sit for it does bring down the overall passage rate though. Schools like Stanford aside, overall pass rate by school is in the mid-high 80s for any Tier 1 school for CA bar exam, whereas for a jurisdiction like NY, those same schools will pass 95%+.
It's well-known to be a harder bar exam (I have taken both it and NY, for an anecdotal data point).
Another reason the CA pass rate is lower than other states’ is that in CA you can take the bar exam as many times as you want. NY cuts you off st 3, as a comparison.