Well microsoft first launched the 64bit support with windows XP and still most of the softwares we use on the 64bit Windows versions of XP/VISTA/windows 7 operate in 32 bit mode . Though the 64bit technology has matured very well with latest windows versions, but there are very few softwares out in the market which uses the full power of a 64bit OS. With a few notable exceptions like adobe Photoshop most other softwares run in the 32 bit mode within 64bit Windows.
But Microsoft isnt worried much, they have already started working on the 128bit technology support for next generation of windows operating system. A profile of a Microsoft Researcher available on linkedin brought to light Microsoft's windows 8 and windows 9 along with the 128 bit support.
Right now 64 bit Windows are only mainstream in the server market where the use of huge amount of RAM has mandated the use of 64bit windows servers. Well the simplest reason for the 64bit OS is that any 32 bit windows cant really fully use RAM above 3.2 GB and hence if you want to have RAM above 4GB then you need to have a 64 bit OS to get maximum use.