CP/M: The only reason anyone consciously bought a Z80 processor and a floppy drive in the 1980s.
OS/2: One handful of satisfied users can't be wrong.
AIX: If Jurassic Park was running AIX the phrase "It's UNIX! I know UNIX!" would have been the last thing anyone heard before the dinosaurs ate the remaining cast members.
Solaris: /opt is a terrible name for a directory so let's put everything there.