Apple announced the Watch yesterday and the didn't call it the iWatch. For starters there are too many trademark issues surrounding the iWatch name to go there, but to me there are other reasons.
1. Apple wants to be in the Watch business
2. Apple wants to sell the Watch via normal watch channels, further expanding distribution
3.By calling it the Apple Watch Apple get the name Apple into all conversations and press coverage surrounding the watch product line.
4. Apple got to include the into every discussion as well now.