They may have wanted California's Supreme Court to make some Californians pay some of the others' insurance costs. Also, they may have wanted to make working in Los Angeles and Silicon Valley more attractive. Few free enterprisers would have supported this proposition. Proposition 103 was immoral. It would have effectively outlawed profit.
Profit is a return for one's goods or services that exceeds their cost.
Profit is the producer's wage. Only slaves work for nothing, and slavery is immoral. Proposition 103 was a legal fiction. Price controls always fail. Proposition 103 would have set California's automobile insurance rates far below the market level, and that would have dried up the supply of insurance.