California passed it’s AB5 law that changed the classification of many workers from independent contractors to being required to be hired as full time employees.
Many in the trucking industry had operated as “owner-operators” who operate their own trucks under contract to others.
The effect, according to many, is that many industries were no longer able to hire independent contractors. There is a summary of some of the issues in this Reuters article: No SCOTUS review of California law’s impact on trucking industry | Reuters
It is quite possible – and perhaps likely – that the shipping capacity problem was directly created by the California legislature – a problem that many warned of before the Act became law.
There have been many court cases on AB5 and I do not know where, collectively, the effect of those cases ends up in terms of impacts on the trucking industry.