In a nutshell, OpenSource developers (well, most of them) want all parts of the source code open, and don't want to use/deal with/work with anything that does not fit that model. In the case of Xara, the CDraw library that does all of the rendering has not been released as source, and most likely never will be.

Although Xara has stated that they're never going to revoke the license on CDraw which would stop anyone that was using XaraLX from being able to continue using it, a lot of people don't want to work on a project that relies so heavily on a chunk of code controlled by someone else.

As a result, the code was forked so that it could be modified to work using Cairo as the rendering engine instead of CDraw. Development can then move forward because there is a version of xaralx that is completely self sufficient from outside code.