While differences in spending surely have some effect, spending isn't the only way to get the word out. I didn't measure, but I feel pretty confident that no more than 60% of the total communication about Prop 22 that I heard from any source was pro. I think it could have been a minority actually. For every PR firm the pro side hired, there were thousands of unpaid activists on the con side (which does say something positive about the con side!).
I noticed you brought out the "lawmakers can't repeal" line. This is true, but also effectively standard for ballot measures in CA. In fact the default is that you can't repeal or change the law under any circumstances. This was one point that initially swayed me to vote "against", but once I learned that this was basically how all ballot measures work (including ones you support, I'm sure), I swung back to voting "for".
For me, this was a difficult call, but I voted "for" on the basis of the net good for the entire pool of current drivers, admittedly at the expense of the smaller number of drivers who would have been able to drive under a better deal had this passed.
The net effect this would have had on driver welfare is far from obvious, so it always bugs me when people assert that the only reason this passed by a 17 point margin is that people were tricked and confused by the ride sharing companies.