I think the rejection is pretty straightforward... It's a lack of trust. California doesn't have a revenue problem; it's got a spending problem. When you couple that with some well known cases of "taxes on the wealthy" morphing into taxes on the ordinary over time (like AMT), it makes voters suspicious that this proposition was actually for it's stated purpose, and even though slippery slopes are a logical fallacy, people don't always employ logic when voting.

There are other factors too of course, but the bill's sponsors failed to overcome the general distrust voters have of their elected officials and California tax authorities.