Microsoft is attributing the decision to the fact that it hasn't transitioned the Ribbonized Office 2011 user interface completely to Cocoa yet. Microsoft officials are saying they didn't manage to do this because they've been consumed with trying to make Mac and Windows versions of the products more compatible with forthcoming release.
From the post: "Because Apple's frameworks require us to complete the move to Cocoa before we can build a 64-bit version, Office 2011 will be 32-bit only….The largest difference between using a 32-bit and 64-bit version is the memory capacity available for your content. Most users with typical or even larger-than-average document content will not notice a difference in performance. Where 64-bit can make a difference is for people working with huge amounts of data, such as those creating very large Excel files with data in millions of cells, or PowerPoint presentations with thousands of high resolution images"