So something crummy but usable-enough for experts (SELinux?) worse-is-better'd its way onto the Linux scene, and now it has matured enough that on the one hand it can't be displaced but on the other its model is ossified and can't be untangled or simplified. Makes sense.
I love Linux and many of the fruits of its messy evolution, but such fruits are certainly not all equally delicious. :(
And no good system makes it into Linux because it has a huge, well supported one, and some 3 other candidates pushing to get there already.