That's a great example of https://news.ycombinator.com/item?id=15772233 (currently also on the front page), where something appears superior on the surface, but there are good reasons why it didn't "win".

"grsecurity/PaX" didn't show up in the main tree but is alive in a different branch or as patch. Various distributions and users are free to enable and use it upon demand.

It kind of prove Linux kernel is not mono-culture and natural selection and evolution are happening.

I believe the AOSP's kernel patches are also not mainlined - also proof Linux kernel are evolving very healthily.