As I mentioned elsewhere in the thread, CA's labor-pool's own hostility to the film industry (via strikes) also played a big role. They gave the studios the finger, then are surprised that the studios took their ball and went elsewhere.
Also, I see this anti-union sentiment a lot on HackerNews: Whenever unions strike, there is near universal rejection as a strategy. The anti-union sentiment on HN is tiring, especially from the US and UK. Those big strikes in Los Angeles resulted in large pay increases and/or expansion of territory (better payouts for streaming, etc.) for those union members. In short, it worked.
It's akin to the auto worker's unions, but without the same degree of government backstopping for the industry. The auto workers unions are an anchor around the necks of the auto industry, while government bailout dollars are a lifejacket - it's clear California (and more broadly the US) doesn't see the same need to subsidize the unions in TV and film.