The reason there is no Naquadah in our solar system is due entirely to the mass of the star that created our solar system. See, about 5-10 billion years ago, there was this giant star that existed somewhere near here. When that star went supernova, it exploded creating a large cloud of gas and dust that eventually formed our sun and the planets of our solar system.
All elements in our solar system would have been made by that massive star, including the iron and carbon that makes up your body.
Since we know Naquadah doesn't exist naturally in our solar system, it means that the massive star that existed before Sol wasn't massive enough to fuse naquadah, thus we have no naturally occuring naquadah in our system.
However, there is a rather large (137km) asteroid floating around out there that was composed of 45% naquadah. With the need for naquadah to build BC-303s ("Enemy Mine"), you'd think they'd have gone out there and mined that rock already.