They did it because they made motor insurance mandatory and it would be unfair to force someone to have something that the market didn't have to provide.
In europe it's essentially impossible for anyone under 21 to get insurance ( without tricks like being listed on parent's cover ) even though the law says you can drive at 17. Typical rates for liability-only on a 17 year old are $10K/year
Imagine living in a rural part of BC where work and shopping is 20 miles away and the insurance companies all charge $10K for drivers under 21.
Yes it's unfair that as a middle age driver I pay 2-3x as much as I would in europe in order to cover a west vancouver millionaire's kid in a ferrari. But it's also unfair that I'm paying for elementary schools for someone else's kid or subsidizing somebody else's cancer treatment.