Driver issues and binary compatibility issues.
Even if they are minor, would you rather get product A which is supported by an overwhelming majority of software and hardware, or Product B for the same price and features but isn't supported as well.
Future-proofing is rubbish, if everyone has one thing it's not going to go away soon for the average user, and 3GB+ of memory is not necessary for 99% of users.
Or, of course, you could get Linux because it's free and tends to have as good 32-bit as 64-bit support because you have access to all of the source code.