I think the reason it's illegal everywhere is with this prohibition, the mobsters/profiteers are narcotic officers, lawyers and prison guards, all government employees. Except for a few dealers everyone in the racket is the law. And arguing democracy would change the law doesn't really work. You can vote bills in but the government can just pass laws without voters. You can vote someone in like Obama who you think will decriminalise and he won't do a thing. It's a lot harder to decriminalise with a vote than to legalise* through a senate composed of mobsters, as california knows.
The other reason is naivety, people who think they're in danger by people who smoke, merely because these smokers are criminalised.
One other reason might be the "I'm more natural than you" argument of the temperance folk, which doesn't hold water considering the close relationship mj users develop with their plants and the environment as a result of consuming it. Whether by growing it themselves, eating, smoking it, or it's natural effects of rendering the scenery more awe-inspiring, or just by being more life that we interact with.
*to make illegal
The insane illegality of pot is the inverse of the sanity with which the general populace regards/indulges in it.
And I'm all for blanket decriminalisation aswell, I'd rather get coke and opium directly from the plant, primary production style, than from the legalised cocoa bean (a monopolised industry in itself, you won't find cocoa butter in the confectionary aisle) and chalk-and-chemical painkillers (codeine is refined heroin is refined morphine is refined opium (which probably has some natural beneficial compounds)). I've known people who took 20 pills a day recreationally, which benefits companies more than people.