Steve Langasek of Canonical notes that due to increasing bug reports, Wubi is not in very good shape for having a release. The Wubi 13.04 installer fails to detect other ISOs, the 13.04 installer isn't creating user accounts, and isn't even working with Microsoft Windows 8. Rather than fixing it, Canonical wants to junk the work.
Langasek says, "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."
This information was shared on the ubuntu-devel list.
A similar discussion about discontinuing Wubi in Ubuntu happened one year but ultimately it was kept around. Back in October, developers were even planning to make improvements to Wubi for 13.04, which never ended up coming to fruition.