Yeah, poseability is a factor. Also the availability of vehicles and weapons without needing to first glue them together and paint them, as in "ordinary" model and diorama making, which means there's less of a threshold to get into it. This combined with the aspect of realistic militarism make GIJoe dioramas the "light" version of model and diorama making, as opposed to, you know, the guys that recreate scenes from the Battle of the Bulge or the battle of Waterloo.