Allow me to play The Devil's Advocate: Those aggrieved by the lack of a Linux driver ABI have indeed forked the kernel, and some of them even maintain their forks. They just happen to be focussed solely on their own needs and thus their forks happen to be specific to their business requirements alone. Take Qualcomm's CodeAurora hosted repos for one: maintained for years, with backporting and new releases based on new upstream releases, all merged with Qualcomm's drivers for their own hardware.

The reason no one has left Linus['s repo] in the dust is because no one has taken the effort to put out and maintain a general purpose API-standardised fork instead of heavily-customised, specific-to-their-own-business codebases.