It's the "porn discount" in action. People won't play a decent VN or an RPGM game just because it's decent. It must have something to offer, something that sets it above the competition. But if it's a porn game, people will play everything that is below average or better.The story apologists and rationalizers will tell you it's all very serious, very revealing, that there's an underlying plan, and a simple explanation for everything that is both logical and forward-thinking. Still, obvious lapses aside generally it's a fairly decent storyline. You need only be comfortable in suspension of disbelief mode for certain aspects of it.
Of course, Eternum is way above average for a porn game. It actually has a plot and memorable characters. Production quality-wise it's better than most non-porn VNs out there.