Hal Berenson defends Microsoft’s strategy for Office on Windows 8:
Would the Office team even have had a stable enough environment to create applications as complex as Word and Excel with Metro in time for Windows 8 RTM? Would they be full recreations, or new subsets (like OneNote MX that doesn’t have all of OneNote 2013′s functionality)? Maybe more importantly, at the time Office 2013 planning was going on did anyone really think that Tablets would have as strong a focus as they do today? (They didn’t.) Or did anyone know that Windows RT wouldn’t be supporting desktop apps in general?
That’s the reason some of the built-in apps in Windows 8 are scrambling to add features now (Mail and Music, most notably). You can’t build full-featured robust apps until the underlying platform is solid.
There’s a ton of other good stuff in this post. If you’re a Microsoft watcher and you’re not reading Hal’s blog, you’re missing a lot.
Ed Bott 
Office is large. It is also–if I understand correctly–overwhelmingly COM-based. That’s not a recipe for quick conversion to a moving-target WinRT. Especially when the Office team was busy with 2013, which isn’t a small update.
Sadly, WinRT is still a moving target, although it’s probably slowing down.
–John
I think this is also a reason why apps are so slow to the Surface RT environment alltogether. Metro programming is really different than Windows programming. So much so that there will need to be a lot of wheel-reinventing going on. This is not a bad thing, give time. Not so in that the apps should be much better overall once re-written. We tend to not allow for time these days though.
Yes, Office is mostly COM-based. Actually, if I recall correctly from Raymond Chen’s blog, Word and Excel don’t actually use real COM, but a custom implementation that predates COM. It’s interchangeable, though, and has been tested exhaustively.
But I don’t buy the “moving target” and “Office team was busy with 2013” arguments. Yes, Windows RT is a moving target, but there are ways to manage around moving targets. For example, Microsoft handled Windows 95 by putting out a “sidegrade” of Office, rather than a feature-laden full upgrade. Office 95 concentrated on compatibility with Windows 95, and introduced very few features.