The arx applications must be recompiled to work with ZWCAD which is great if you happen to have the source code that created the arx.  Most companies are using 3rd party ARX applications and as such, do not own the source code to convert for use with ZWCAD and/or iCAD. 

Many third party developers for Autodesk (e.g. large applications like HydraCAD or SprinkCAD) are not interested in recompiling their applications for use in non-Autocad applications.  This is obviously short-sited but unfortunately, that is their narrow view of the present market.  The third party developers who are flexible enough to convert their apps to run on all platforms will ultimately derail these short-sighted mentalities but that will take some time.