The simple answer is that prone isn't always more stable than sitting, and sitting isn't always more stable than prone.
The difficulty level of the prone position is a decision made by the course setter on a lane by lane basis.
If sitting were allowed, then one of the course setters 'tools' would be taken away from them.
Oh and there has actually been at least one sitting shot in a national UKAHFT event so it's not banned completely.
Something common to both sports (and they do share lots of commonalities) is the banter and pis
s taking. That doesn't always come across too well on a forum though, some of these posts read a bit like you actually mean it rather than just friendly banter
You should be careful with threads like these - if you damage one sport you damage both sports.