Generally, distinctions of "employee" versus "contractor" come down to things like the level of control exercised over a person's activities, when and how they perform those activities, etc. It appears California found that those factors tipped more to the "employee" side.
And it may well be that if taxi drivers sued at least some of them would also be ruled employees. Until then all we can say with certainty is that taxi companies claim their drivers are contractors.
Most taxi drivers essentially rent out the car and medallion each day for a set fee, after which they are free to do whatever they want including just idling all day. Many of them aren't even contractors, they're sole proprietors or operating under their own LLC. Uber drivers are beholden to the app at the risk of being dropped from Uber all together, which ticks a few boxes on the legal guidelines for how to classify someone as an employee vs contractor. The California Uber case is particularly enlightening.
Taxi drivers who do it that way sign a contract with the owner of the medallion but that does not make them contractors just like signing a mobile plan contract doesn't make you a contractor for Verizon or TMobile. They rent the car and medallion so that makes them renters.
And it may well be that if taxi drivers sued at least some of them would also be ruled employees. Until then all we can say with certainty is that taxi companies claim their drivers are contractors.