This is the real reason in my experience, combined with the bringup/support cycle that comes with hardware development. If you fork the driver for bringup, you

This is the real reason in my experience, combined with the bringup/support cycle that comes with hardware development. If you fork the driver for bringup, you buy yourself a lot of freedom to change things. During support phase, when the hardware is already in the hands of the customer, changing things is more risky and so you have to be conservative. For example, maybe you had a chip bug in the previous hardware generation which caused the system to hang after several days of stress testing. You found a software workaround for the bug, but every time you touch the code you need to re-verify the workaround, which takes days. Of course, the downsides of forking the code are also very apparent...