Microsoft needs to complete the transition from Carbon to Cocoa before it can build 64bit versions of Office, since Apple decided not to transition Carbon to 64bit. While this effort is currently underway, the MacBU states they won't be able to complete it because they decided to focus on other matters first.
"In Office 2011, we've made investments in better compatibility between Office for Mac and Windows Office, which is the largest request we receive from customers," writes Jake Hoelter, Product Unit Manager at the MacBU, "Our work to increase compatibility means we haven't completed the transition of moving the entire user interface over to Cocoa yet. And 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."
Several parts of the transition are already completed, though. Entourage has been rewritten in Cocoa from the ground, and has been renamed to Outlook. In addition, all the new interface work (the Ribbon) is written in Cocoa too.
"Looking ahead at the MacBU development roadmap, we expect to go even further with Cocoa in the future," Hoelter adds, "Meanwhile, I think customers are going to be really happy with the improvements in compatibility, collaboration, and user experience we're bringing in Office 2011."
This news isn't particularly surprising. When push comes to shove, the Macintosh platform only accounts for about 4-5% of the world's desktops, so you can hardly blame Microsoft for not devoting additional resources to spur the transition to Cocoa if their users indicate other features are more important to them.
On top of that, while Apple's Steve Jobs derided Adobe for being late to the game when it comes to Cocoa, the fact of the matter is that Cupertino itself isn't really setting a good example either. iTunes is still 32bit Carbon, as is Final Cut. It's interesting how Jobs derided Adobe for being the last major third party developer to adopt Cocoa, leaving out Microsoft, while in fact Apple itself hasn't completed the transition either.