Sometimes drivers opt out because the hardware is odd enough that it needs to do things differently, or because it offloads to hardware (or proprietary firmware) functionality that is usually done in software. But often, it's just because a driver was written in isolation by the manufacturer and then dumped on the community. (See [2], from the comments in [1], about the work required to clean up some Realtek WiFi drivers enough to be merged to the kernel staging area.) If a driver unnecessarily opts out of common frameworks and does things internally and differently, it can be hard to even evaluate whether problems fixed in the standard frameworks exist in the special snowflake drivers. Even after identifying a problem, the recipes that worked to fix the standard drivers won't apply.
[0] https://lwn.net/Articles/454390/
[1] https://lwn.net/Articles/705884/
[2] https://www.linuxplumbersconf.org/2016/ocw//system/presentat...
Proprietary drivers are tolerated, not liked, and people aren't interested in making it easier for them.
That said, the community isn't afraid of breaking changes to push future versions forward.
Linux doesn't follow SemVer.
