Win64 won't work with 32bit drivers. That means every single driver out there needs to be reworked and recompiled for x86-64. That's not going to happen until every single manufacturer has had a chance to update its drivers. Microsoft isn't going to do all that itself. That means all of those manufacturers need x86-64 machines to work on; probably lots of them.If Hammer supplies are tight, it seems plausible that there wouldn't be enough driver development systems to go around. Since Microsoft would likely require a complete driver library before releasing a 64-bit version of Windows, especially one for desktop users, the Athlon 64 would be left without a desktop operating system.
Unlike the Athlon 64, server-targeted Opterons can get by with a 64-bit version of Linux. Server customers should be more comfortable with third party drivers and even recompiling drivers for 64-bit compatibility themselves, so a 64-bit version of Windows isn't essential.