Elementary, my dear Watsons. First, propositions require yes or no votes. I have said for years that success will come in legislative action first, as it is doing, because legitimate objections can be reconciled with amendments. Remember that even Howard Jarvis, with his much less controversial Proposition 13, which limited property taxes and passed in California in 1978, had to try several times before he got the wording right. One day, when they get the wording right, and can muster sufficient financial support, backers of school choice initiatives will also begin winning.
Initiatives are political campaigns, so money counts most of all.
Those who oppose freeing the children of the poor from the economic prisons their schools often are, lacking truth and justice on their side, pour on the money, the negative advertising, and the scare tactics that too often decide how votes are cast. This has worked, and it may work a bit longer.