Intel Logic

Oy vey! The Z80 has built-in BCD instructions! Better add those to x86! 8 bits at a time is good enough for Z80, it's good enough for us!
Oy vey! Floating point is popular! Can't let those third-party accelerators beat us! Better rush out some notoriously difficult to program stack FPU turd!
Oy vey! VAX has 4 security rings! Better make sure we have 4 too so OSes that use them all are hard to port to RISC!
Oy vey! PowerPC has AltiVec SIMD instructions! Better whip up some half-assed shit called MMX and pretend it's innovative!
Oy vey! MMX is half-assed shit! Better make an improved sequel called SSE! Follow that up with AVX! If Hymiewood can sell the goyim the same shit over and over again, we can too!
Oy vey! Real computers have VMs! Better add some privileged VM instructions so hypervisors are forced to run in kernel mode!
Oy vey! I think I missed a few ``innovations''!

http://en.wikipedia.org/wiki/AVX-512
The VEX prefix used by AVX and AVX2, while flexible did not leave enough room to for the features Intel wanted to add to AVX-512. This has led them to define a new prefix called EVEX.
Kek. Intel never thinks ahead. Ever. Not with 8008. Not with 8080. Not with 8086. Not with 8087. Not with interrupts. Not with A20 (sure, blame IBM because Intel's CPU let memory wrap around like that). Not with 286 Protected Mode. Not with 32-bit mode. Not with MMX and SSE. Not even their ``innovative'' AVX.
The original 8086 was never meant to be extended so when they came out with 286 they turned POP CS (0x0F) into an extension prefix. Any sense of regularity just went downhill from there and was replaced by ``stick this instruction in there so we can say we have what our competitors have''.