Ed Kilgore attempts to diagnose the proposition's slump in the polls:
One possible explanation for the polling trend is that Governor Arnold Schwarzenegger’s apparent effort to undercut Proposition 19, by pushing through legislation that all but decriminalizes small-scale pot possession, has worked. This new law, which Schwarzenegger signed on September 30, makes possession of under one ounce of pot an “infraction” punishable by a $100 fine—significantly less than the average California speeding ticket. This may have deflated support for Proposition 19 among voters who are less motivated by the desire to fire up a doobie themselves as by concerns about the injustices caused, particularly against minorities, by criminal sanctions on the use of marijuana.
Mark Kleiman shows why decriminalization isn't enough:
Possession arrests are already infrequent, except in connection with other offenses. But that’s partly because a custodial arrest is a fairly expensive process from the viewpoint of the criminal justice system. By contrast, writing tickets is cheap: probably a source of net revenue. In one of the Australian states, decriminalization actually increased the number of cannabis possessors going to jail; there were far more tickets than there had been arrests, and some of the people getting the tickets didn’t pay the fine and went to jail for that.