From the blog post:
The decision to use the name Windows 7 is about simplicity. Over the years, we have taken different approaches to naming Windows. We’ve used version numbers like Windows 3.11, or dates like Windows 98, or “aspirational” monikers like Windows XP or Windows Vista. And since we do not ship new versions of Windows every year, using a date did not make sense. Likewise, coming up with an all-new “aspirational” name does not do justice to what we are trying to achieve, which is to stay firmly rooted in our aspirations for Windows Vista, while evolving and refining the substantial investments in platform technology in Windows Vista into the next generation of Windows.