Because we have a 32 bit UEFI, so 64 bit systems are not supported !
32-bit Windows (x86) is installed on 32-bit UEFI. 64-bit Windows (x64) is installed on 64-bit UEFI. This is part of the UEFI specification, which dictates that the underlying firmware match the OS runtime (easier for firmware interfaces).
And you cannot replace the UEFI firmware!