It is a long story, that I do not remember entirely and corretly, but it was mostly politics...
Part of it was because OSSv4 was briefly not open source, and this pissed off lots of people, when it became open again, there was some political fights (that I don't remember the reasons), and there is some technical details of OSSv4 that irritate the kernel people (like the fact that instead of being entirely on userland it rely on Kernel space to reduce latency, specially for games, and the kernel people say they believe it should be userland only).
Part of it was because OSSv4 was briefly not open source, and this pissed off lots of people, when it became open again, there was some political fights (that I don't remember the reasons), and there is some technical details of OSSv4 that irritate the kernel people (like the fact that instead of being entirely on userland it rely on Kernel space to reduce latency, specially for games, and the kernel people say they believe it should be userland only).