It's called free-ridership.
The US Constitution says that California can't keep out any American, and has to treat them the same as people who were already there. Basically immediately.
If California implements non-deny universal coverage, healthy people will choose to work in states without health care mandates. Then, when they get a cancer or other diagnosis, they hop the first plane to California and establish residence.
Virtually all countries have a medical exam as a part of immigration. Unless you want to amend the US Constitution so that California can deny access to incomers who already have medical conditions, it's an intractable problem. Home insurance can work on a state-by-state basis (the house doesn't move). Even car insurance can (the car moves, but accidents are discrete events that happen in one location or another). Medical insurance doesn't.