Off the top of my head:
Nobody questions why main drive is C:, remnant of [an] early computer having two floppy (not sure) drives on A: and B:
Or more recent - C:/Windows/System32 holds 64 bit executables; 32 bit exectuables live in C:/Windows/SysWOW64
You can call the 64 bit architecture x64 all you like, but it's still using the x86 instruction set and it's frequently referred to as x86-64, so naming that 32 bit only folder "... (x86)" will just make things more confusing than they should be.