No one answer covers it all. In part, what was alluded to earlier on the question of transport, when a number of young people hit the age of 16, they've had it with being on a bus sometimes three to four hours a day. When they leave home, they're tired; when they get home, they're tired. School means they're in effect excluded from many of the extracurricular activities that build friendships that lead to stability in a community.
Another factor that has had a major impact is that when regional schools were developed, secondary schools were closed in many of the communities, and the students were bused away. It failed in one important aspect, not in terms of its educational goals—there was much more variety—but it meant young people no longer found themselves rooted in a community, in a small town, or on a farm They saw themselves essentially in transit, and as soon as they reach the age of 16, there is an imagined paradise that exists somewhere out there. It's west and it's bigger and it has so much more to offer.
If we went back 100 years, many jobs were available that didn't require specialized training. That is no longer the case, so dropping out happens.