Well that, and the fact that we already have plenty of OSes to run on x86(-64).
Looking at arch/ in linux's source:
alpha avr32 frv Kconfig microblaze openrisc score um
arc blackfin h8300 m32r mips parisc sh unicore32
arm c6x hexagon m68k mn10300 powerpc sparc x86
arm64 cris ia64 metag nios2 s390 tile xtensa
I'm also surprised that I can't see mention of Z80 in GCC's documentation.
I'm somewhat surprised there's no Z80 support for GCC, I recall running a gcc for Motorola's 68HC11 which is a similar processor. That said, most general purpose C compilers are a bad fit for 8-bit processors; you really want to write assembly for these small systems to ensure your code is compact and fast; it's much too easy to write code in C that will be significantly slower than if well written in assembly because of limited memory indexing modes or lack of registers. It's probably possible to carefully constrain your C usage to encourage reasonable assembly output, but then you're not going to be able to use existing C code. You won't have that much code that fits in your roms anyway, so you may as well dig in and write assembly.
The was only introduced with the 68030, which is roughly analagous and contemporaneous with the Intel 80386.
The 68040 added an onboard FPU, like the 80486. (And like the 486SX, the 68040EC had the onboard FPU removed again.)
That's because it's an 8 bit computer, my dude. Back in the old days, when "the internet" was still a military project, and you'd phone up your local BBS at 2400 baud on a POTS with suction cups; that's all the little people had access to as recently as the early 80s. And as other people said, there are apparently many of them around, and they run on shit power supplies.
It's a cool idea, but obviously it requires both cheap and dirty hardware implementations and a paper manual. Pretty sure "hacking" will be low on the hierarchy of needs in the event of apocalypse. Also pretty sure something like CP/M would be more useful. I know where there are CAMAC crates with Z80/CPM punched card/paper tape readers that would probably do great in a post apocalyptic environment.
I wonder if he knows about them?