The data available on the ODB port is a combination of the data required by the law in the country where the car is sold and the data that the vehicle maker wants to expose for diagnostics and tuning.
It's worth pointing out, I think, that the PIDs aren't necessarily there and not exposed in all cars. Vendors choose what data to expose and in many cases the data simply isn't there or doesn't make sense in the context of the particular vehicle (e.g., automatic transmission related data on a car with a manual) or steering angle on a car without a steering angle sensor. The list of standard PID values exists to help bring order to the industry and assist regulators and repair shops – it's not a list of what can or should be implemented (except in the case of the data that is legally mandated).
Beyond the data that is mandated by the local authorities, the data and the tools to read the data are often proprietary – or maybe it would be better to describe them as "not publicly documented," since in this day and age any vendor who wanted to truly protect their data and interfaces would probably use encryption. Arguably they should be using encryption to help guard against attack.