This was a legalization measure that, before anyone else much cared, split the pro-pot base. Many commercial (grey and black market) growers were agin' it (fearing a market crash). Many medical pot providers were agin' it (fearing being taxed to death and/or getting shut down in the ensuing chaos, stranding needful patients either way). In "the community" of casual tokers, BS rumors were all around about various ways in which Prop 19 was eeevil.
So, right off the bat, the base was split.
Then, for everyone else, comes a lot of other BS like "this will legalize driving while stoned; this will legalize getting stoned at work; etc. And all because -- while legalization might be a fine idea -- this law is badly flawed."
Well, actually, a lot of that BS was pretty far off the mark but it was convincing enough to scare away a lot of voters who might otherwise have been fine with legalization.
NOBODY on the yes campaign made the serious and needed case about lowering the incarceration rate, about undermining organized crime, etc.
Frankly, I never saw any serious message from the "yes" campaign. I couldn't tell you their talking points if I wanted to. Meanwhile, the "no" campaign's are easy to tick off one by one.
Prop 19 lost because the "yes" campaign was very -- amazingly -- incompetently run.