I would bet that the readiness/ability of the people who attempt the apprenticeship track is much lower than those who attend CA law schools. This may be the reason that so few people succeed via this route — not because it's harder.
Most people who take the apprenticeship track are pretty exceptional because they have to convince a practicing lawyer to essentially hire them with zero experience. The lawyer-apprentice relationship is very hands on, not like an intern or even a paralegal.
Regional law schools tend to be more focused on black letter law, partly because their students tend to be less capable of cramming all the black letter law in the 2 months before the bar exam.
But it would not be difficult at all for someone who had apprenticed for a lawyer for multiple years to take a Barbri class and pass the bar.
The main issue is the prestige factor. People will assume that you couldn't get into a decent law school if you went that route. Perhaps some students with sterling undergraduate credentials could try to break through and get jobs based on their undergrad connections.