Aside from this thing office has always been the de facto, but it wasn't until office 2007 (ribbons? I don't recall if it was 2007 or 2003) that the office suite was revamped from an UI point of view. That really was the moment microsoft made a difference that wasn't matched (and still isn't). Before the 2007, even though the open office suite was inferior because of the compatibility issues, the interfaces weren't so superior (IMHO).
The ribbon bar (loved or hated) was the first step towards the recent UI improvements. The same OneNote has vastly improved.
Also, I am specifically not talking about feature-richness. It's obvious that there are not so many competitors to office in this regard, but still with many features comes complexity, and complexity comes at a price. Microsoft found a great way to reduce that complexity without sacrificing features. (a totally different way compared to apple, which _usually_ start with less features and adds them over time).