I guess that is due to the fact that IFCSite (http://www.buildingsmart-tech.org/ifc/IFC2x3/TC1/html/ifcproductextension/lexical/ifcsite.htm) is an spatial object in the IFC structure, which means it is part of the building structure, just like IFCbuildingstory (structural level story in Allplan) is. I think the object is not exported because the IFC does not allow the object type IFCSite (structural level Site in Allplan) for a physical object, but only for a structural level in the building structure.