I have a feeling it's because ActiveSync 4 made big changes behind the scenes: instead of using a USB / serial protocol as did ActiveSync 3.8 and older for WM2003, it uses networking protocols. However, WM5 has been out for quite some time and I don't know why the folks who make ACT haven't updated their software yet.
Have you ever been able to sync the WM5 Dell to ACT?
-------------------- Lisa Gade Editor in Chief, MobileTechReview