My speculation, and I think this was backed up by some of the Governor's commentary in the press conference last week about "smaller" amusement parks, is that the state knows that Disney and Universal will adhere to any guidelines approved by the state, but that it will be a free-for-all at other parks.
There is no way that a Great America or Six Flags staffed by seasonal teenage workers that provide a sub-par guest experience already is going to implement and enforce rigorous safety standards. Even in Florida, the reporting that I've seen has been that adherence to safety requirements has been lackluster at Sea World,
Legoland, and regional parks. Has that led to outbreaks yet? No, not that we know of. But I don't see Governor Newsom approving an opening plan that he knows some parks won't follow and any plan that says "theme parks that charge and make a lot of money can reopen but other theme parks can't" is not legally defensible.
Part of this could just be a waiting game -- all of the water parks already gave up on 2020 and have moved on to 2021. This is the time of year when some regional parks move to weekend-only operations, so if the Governor keeps pushing off reopening, they may just decide not to reopen until summer 2021, which may make it easier for the Governor to move ahead with allowing limited reopening, knowing that practically speaking only some of the big parks (like Disney) will choose to reopen.