How to Build a Cosmic Empire in the Middle Ages

Why was no one able to put together a coherent successor to rome in the west, whereas in the east you do get very successful, gigantic empires? I suppose perhaps one argument would be the prisoners of geography approach. But if we think about what had to be done in order to hold this area together under imperial rule, it required military spending on a scale that dwarfs what the americans spend on the military to day. Such extreme fortumity of conditions and then extremity of methods that all empire building is really quite difficult to do and much easier to undo than told plantagenet family, too nice.