Because obtaining it illegally is a pain in the ass. If I can just go down to the gas station and pick up a soda and a sack, you can bet that's what I (and most people) will do.
Sure people still make moonshine illegally, but it pales in comparison to legal alcohol sales, simply because it is so much easier to just do it legally.
There's a difference, though. Making moonshine is still illegal. Under prop 19, adults have the right to grow their own cannabis in a 5'x5' plot, and keep and share (share, not sell, but that's not much of an obstacle) the harvest.
More on topic, though, I plan to vote YES. As has been said before, the prohibition of cannabis does much more harm than good. Prop 19 will do the following:
1. Increase tax revenue. California is broke, and we need all the tax revenue we can get. Cannabis is California's biggest cash crop, with an estimated $14 billion in sales annually. And right now, only a tiny fraction of that is taxed. While this initiative doesn't include a tax, it provides a framework for the state legislature as well as individual cities to levy taxes on cannabis.
2. Decrease law enforcement spending. California spends roughly $200 million enforcing prohibition every year. Our jails are full, our courts are packed, and again, the state is already broke. This frees up a ton of state funds to be used for more important things, like education.
3. Weaken drug cartels. Much of the illegal cannabis trade in California can be traced back to Mexican drug gangs. Remember that $14 billion Californians spend on cannabis every year? Yup, a lot of that goes to violent gangsters over the border. We can draw parallels to alcohol prohibition, which put power in the hands of bootlegging gangs.
4. Decrease racial ineq-**** this. I'm too high for this ****.
Smoke weed every day.