It may be a very good reason to get away from it, but Windows XP still has almost double the marketshare of non-Windows operating systems. That includes OS X, which Kobo also supports. (The figure is based upon web analytics, which is probably all that matters here since embedded systems, servers, and non-connected computers are outside the scope of Kobo's market.)

Another thought though: Kobo Desktop (at least on the Mac) is based upon Qt5. There was some discussion in another thread about Qt5 not (officially) supporting XP. That probably has some bearing here too.