This one seems obvious since Microsoft is a powerhouse in the gaming world and they constantly have new announcements and demos that they would naturally want to show off in a convention setting. Microsoft has a loyal fanbase and they would have no problem getting people to attend. The convention could sell merchandise, show off new console features, and maybe even have developer-themed panels. Plus, their biggest rival Sony has their own convention, PlayStation Experience, which offers a mix of fan and press events. If Sony can have one, then there is no reason for Microsoft to stay out of the action.