Yeah the main story - its consistently comical narrative is the success story. I agree because there wasnt much sex going on back in the day as i for anyone be that Sterling OR Hunter so dont pretend it was because it was a harem game. The game was a slow burn in that regards (with only Eva offering any action).
The NTR undertone was a massive contributor to the success none the less though as was its other big taboo - Incest. Those underpinned the hormonal long term investment whilst the storyline made it stand out from the 100s of other harem games out there with surprisingly enough no shortage of family fun too so even that is not exactly its USP either, as said it was the fact it was so fucking brilliantly funny in its writing that it won people over and grew to the success it is now (look how many games are created and fail on this site - few get to rise to the top and be successful standing out from others).