Compatibility
There are some "minor" software incompatibilities between the 64bit and 32bit versions of Windows... most notably, old 32bit drivers. If you have any old "legacy" hardware in your PC for which there is no 64bit driver, your only option is to replace it with one that does. 64bit versions of Windows do not support 32bit drivers. No way, no how.
(and I have plenty of legacy hardware, so upgrading would be costly.)
The advantages of 64bit FAR outweight the advantages of continuing to use the old 32bit platform, but I won't upgrade (I still use 32bit XP Pro) until MS abandons the 32bit platform entirely. Software developers will continue to write for the Lowest Common Denominator to maximize their sales until MS gives them and everyone else no other choice but to upgrade.
Until then, I'm sticking with my trusty & reliable 32bit XP.