According to Mike Nash of the Windows Vista Blog.
Mike Nash said:
As you probably know, since we began development of the next version of the Windows client operating system we have been referring to it by a codename, "Windows 7." But now is a good time to announce that we've decided to officially call the next version of Windows, "Windows 7."
The decision to use the name Windows 7 is about simplicity
Now we know why MS didn't go ahead w/ that $300 million Vista campaign.
It makes sense since "Vista" has been poisoned. Maybe they could've saved "Mojave." Nah. I think "7" is good. It's a good decision to start a new brand. Who doesn't love a lucky 7?