* SSE and other architectural extensions combined with supporting patents
* Intellectual property minefields about how to adapt every new micro-architectural innovation to the mess that is x86 architecture.
Finally, barriers to entry in developing a competitive processor are fundamentally high. Combining them with the risk of litigation makes it untenable.
Why up to this day people still can't make x86 clones? The amd64 architecture is nearly 20 years old, and lots of compilers target ancient processors.
What kind of legal protection is there that is lasting for that long?
(Anyway, I'm glad RISC-V is taking off.)