1. To avoid Linux' geeky image. Don't forget that many potential users previously had bad experiences or heard about them. Linux on the desktop has an image as a tinkerer's system.
2. To elevate themselves to the 'operating system' rather than 'distribution' category (i.e. one can pick between Windows, OS X, Ubuntu, and Linux). This gives the perception of Ubuntu being a real competitor to Windows and OS X, not just another flavor of a marginal (on the desktop) operating system.
I think that from a PR-perspective this is a good thing.