Because I have *zero* experience using premade CPU cores.
I had a great idea for a CPU design a while back and started building it in VHDL. I have been perfecting it for the DEXT Meteor project recently.
The architecture has 8 32-bit registers, including the stack pointer and program counter:
|3||DX (can be used as address)|
|4||SI (can be used as address)|
|5||DI (can be used as address)|
The architecture and instruction names are loosely based off of the Intel 8086, except 32 bit instead of 16 bit. The instruction set, however, comes from the Z80, except there are no extensions. There will be an ldir instruction to load memory, but also instructions like lodsb and stosb from the x86 series of CPUs.
I call it the Plasma Cortex (because plasma sounds really cool), and currently I am implementing a cache and better memory controller.