You can read more about this decision here. Users who wish to try out Ubuntu without repartitioning a Windows system are encouraged to use a live system instead, booted from either a DVD or a USB disk.
Recent bug reports suggest that the Ubuntu installer for Windows, Wubi, is not currently in very good shape for a release:
13.04 installer doesn't create user account
Wubi fails to detect 12.04.2 and 13.04 AMD64 ISO
Combined with the fact that Wubi has not been updated to work with Windows 8 (bug #1125604), and the focus on mobile client over desktop, the Foundations team does not expect Wubi to be in a releasable state for 13.04.
I am therefore proposing to drop Wubi from the 13.04 release, starting immediately with the upcoming Beta. This will save our testers from
spending their time testing an image that will not have developers working on fixing the bugs they find, and spares our users from using an image for 13.04 that is not up to Ubuntu's standards of quality.
If someone is interested in taking over the maintenance of Wubi so that it can be released with 13.04 (or if not with 13.04, then with a future
release), I would encourage them to start by looking at the above mentioned bugs and preparing patches, then talking to the release team.