If the constitution says don't do something, and people vote for legislation to do it, then the constitution has to be the controlling document or else it doesn't mean anything.
Also in CA judges are elected by the people.
The rule that allows judges to determine if a measure presented to the people met the rules for such votes, and the rule for the votes that judges apply, were, themselves, adopted by a democratic vote of the people.
Laws are not last in, first out; previous laws need to be taken into context when new laws are created, no matter how they are created. The fact that this law was created by popular vote has no bearing on its validity or standing in court.
There are important exceptions, see "tyranny of the majority."
Uber and Lyft were threatening to simply shut down if this proposition lost. A LOT of people at the time only had Uber/Lyft as their income. That was going to be catastrophic.
Trump was still President so people had little prospect of unemployment benefits. We had no vaccines and no known timeline. Lockdowns were still in effect at various levels of strength.
If you had this vote again, now, I'm not sure it would pass.
(I could also go into how Uber and Lyft were spamming the hell out of my email and phone. That should have gotten a whopping fine from the FEC.)