Just a guess on my part but I think the 5th wheel hitch puts less strain on the trailer hitch point. This would allow the manufacturers to put together a lighter, some would say poorly, designed trailer.
OTOH I know some people that have used a gooseneck adapter to pull a 5th wheel trailer and have had no issues. My BIL pulled his with an adapter and pulled a pretty good sized boat behind the 5th wheel and has had no problems. IMO the leverage created by the adapter seems like it could definitely create problems if the hitch area of the 5th wheel is marginal.