Although Xubuntu is an official flavor of Ubuntu, it is community-supported rather than managed directly by Canonical.
Projects such as Xubuntu are independent community-based projects and do not necessarily follow everything that Canonical does with the mainline release.
The simple reason for not creating images for Macs is that it is not part of the project's goals, plus they are unlikely to have the resources or volunteers needed to create such images.
This is not the only area where the community-supported flavors differ the Unity release. For example, Xubuntu 14.04 is an LTS release, but it will be supported for only 3 years rather than 5 years. Similarly, in Lubuntu 12.04 is not an LTS release at all.