I'm not on the scene. I don't work in that business. So take this with some salt. But I have heard that two things are causing issues.

1. California passed a law that all trucks older than 10 years old had to have expensive modifications (I believe for pollution control). Well, the average age of trucks is 14 years. Surprisingly (not), trucks became in short supply in California.

2. They passed this law that Uber drivers were employees, not contractors. Surprisingly (until you think about it), this same law hit owner-operators of trucks. That confuses the legalities of hiring owner-operators, which is a fair number of trucks.

From what I heard, for both of these reasons, trucks and drivers are less available than they used to be in California.