To elaborate a little:
In Japan, a well-written H-game can be redesigned to be anime / comic / light novel / other goods etc, so they can expect a good pay from a game series that have a good name. "Fate" is one of the best example. JP writers have a good reason to keep their stories interesting and not solely relied on sex, so that the story will have a larger chance to be animated.
However, in western world, adult games are completely cut off from movies / TV shows. The developers can only rely on their patrons, and most of them love sex. That's why visual novels are much less popular here.
Also, Japan has a long tradition in making visual novels. Even flash games do have a plot.
