Even as simple as it might be, this is one of the reasons. Japanese do read a lot when compared to other countries, just look at circulations for some of the magazines there.
But as for question of Visual Novels in other countries… Much of the problem is that many of the people who might be interested in them have no idea that visual novels even exist. There’s also stigma of porn that comics, manga and anime had to(and still have to) deal with.
Personally I’d be interested to see how properly marketed visual novel would do in Finland, since people here read even more than Japanese if look at population. One weekly comic magazine has circulation of like 2 million copies.