It was a poorly written law. Support for legalization nationwide is currently about 55-59% in the abstract, but on any given law it's rarely higher than 50%. Prop 19 was an especially bad law for a variety of reasons, and even then it came very close to passing.

What was wrong with it?

Medical dispensaries are opposed to legalization for recreational use because they currently have a state-enforced near-monopoly on Cannabis sales.

Unintended regulatory consequences in three, two, one...