I was browsing the Wubi code the other day and noticed this revision: Drop Wubi for Xubuntu at the request of Pasi Lallinaho. Pasi Lallinaho is the project lead for Xubuntu: https://wiki.ubuntu.com/PasiLallinaho/
Xubuntu is typically used on lower spec hardware so it's probably not a big surprise to see this move. The minimum requirements for Xubuntu typically are lower than that of the Ubuntu installer (ubiquity) so in some cases the Wubi install would fail when a normal Xubuntu install would succeed. But I don't know the real reason behind this move.