I think Phil has hit the nail on the head with the closed/open difference.
I do however see more wargame rules for toy soldier games trying to be more ‘closed’.
I have several rule sets that specify exactly what the size of table should be , define precisely what sectors of the table figure set up must be in and how much terrain should be where – not as options but as part of core rules. They can also define army sizes and define winning in terms or their own army point loss mechanisms. Now I am quite happy to ignore these where my desired scenario doesn’t fit with them, but I suspect that increased commercialism in wargaming is leading us towards a mindset that sees rule sets as ‘complete’ games rather than mechanisms for administering one part of the gaming experience.