Why can wine run 64bit programs on Linux but not on Mac?

The answer is given in a mailing list comment by Stefan Dösinger entitled Wine 64 bit

OSX has a ABI incompatibility with Win64 - OSX overwrites a CPU register that Win64 applications expect to remain untouched. Apple can’t change the ABI because there are already 64 bit OSX apps that expect things to work that way. A potential workaround may be to run Wine inside a CPU emulator like qemu, but that is anything but easy.