The poorest billionaire in California will make about $300 million this year. If we pass the CA wealth tax, he will only make $235 million. In exchange, millions of the poorest Californians will get health insurance. That's the tradeoff billionaires are not willing to make.
Other people didn't mention that it's a one time tax, vs annual tax and that makes a difference. It costs a lot of money to move and you probably rip up longtime deals you had with the past state if your moving business headquarters too.
States could be imposing an annual tax of up to 10% and most of these billionaires would still be billionaires the next year, a 5% tax is an inconvenience... but a horrible precedent (for them)