My guess is that there is a very good reason for that. Our boxes are not compatible with XMBC (nor KODI), due to hardware & software limitations. Remember the hardware is designed for being a receiver and the embedded Linux version is limited.
As a result of that VU modified an existing XMBC version, and also added some functionality via their drivers. Obviously when doing so you're always behind the development of XMBC itself.
IMHO that is not a real limitation, as this implies that there is only one party responsible for the functionality of XMBC as well as the plugins. In other words: image builders just add what they get from VU+ without having to reinvent a rolling wheel. In fact the same as with HbbTV.