The main reason is app compatibility, which is why 32-bit Office 2010 is recommended even on 64-bit Windows. There are very few if any 64-bit add-ins for Office. For a more thorough discussion on the difference between 32-bit and 64-bit Office, please take a look at these docs:
http://technet.microsoft.com/en-us/library/ee681792(office.14).aspx
http://msdn.microsoft.com/en-us/library/ee691831(office.14).aspx
In a default installation of Office 2010, the 32-bit version is installed, even on 64-bit systems. You must explicitly select the Office 2010 64-bit version installation option.