Microsoft destroyed their old blogs and with them a lot of knowledge they contained.

Thankfully, Internet Archive preserved the "Why the new UI" series of posts: https://web.archive.org/web/20080316101025/http://blogs.msdn...

Relevant parts: by Office 2003 they had 250 top-level menu items and 50 toolbars (Part V), and this was becoming unmanageable. In Office 2003 they started collecting info on how people are using Office: About 1.3 billion sessions since shipping Office 2003, over 352 million command bar clicks in Word over the last 90 days (Part VI).

I won't quote the rest because it's a very good series of articles.