I guess it's because OneNote is more of a desktop app that saves data locally and is commercial (I am not personally aware of any EASY option of storing OneNote data remotely - although I wouldn't be surprised if there is one).
I guess the fight is for free and generally useful, stripped-down, online applications that would prove useful to the masses which would, in turn, allow Microsoft to generate ad-revenue and direct users to other service offerings like search, etc. just as Google has been doing for very many years. Thumbtak won't compete with OneNote and even if it does, at this point, Microsoft really doesn't seem to have a choice BUT to compete in the online applications domain lest it be left faaaar faaaaar behind and be forgotten. And like Google, Microsoft may even provide corporate versions of their online apps which provide better collaborative features.