If I had to take a guess, I'd say it's simply because it's quite refreshing to connect at this level with characters. They're well written, the renders are indeed beautiful, and the story leads you to care about each of them in a way that isn't just "when will my landlady/roommate let me jerk off to her sleeping naked body!?".Can someone explain why this has blown up in popularity? Even if it's got above average story and renders it still has limited adult content on top of a clunky UI.
A lot of people might not care for it, but if you're among those foolish enough to appreciate spending good times without the need to bust a nut every two frames, that game hits all the right buttons.
I hope it helps you get the popularity