When Windows debut, it was called that because applications ran in different window panes. You can move them around and have multiple windows on one screen.
So now that Microsoft is using tiles, I don't see any reason why they want to call it Windows. Brand recognition is not a good reason as the reputation of Windows is Luke warm at best.
Time for some thing new. The OS is new, and Microsoft wanted a fresh new start for Windows Phone 7. It's a radical departure from Windows Mobile of the past. So why saddle it with an old name that doesn't invoke excitement?
Microsoft Tiles. I think it has a nice ring to it.