Because no politicians have the balls in this day and age to seriously stand up for marijuana decriminilization. Denver's started, but if other cities start to discuss it there will be a huge backlash and all these "anti-drug" advocates and MADD type groups will start to criticize, make up statistics to try and stop it, and the politicans fear not being re-elected.

I'd give it another 10 or 12 years before decriminilation for weed(and possibly shrooms) starts in the United States.