I_Need_An_Adult For me it's a combination of above average writing and characters, and a unique art style. And then you add into it mechanics that are like 25% as grindy as other titles, along with, what is by now 118 years since the game launched, a fairly good chunk of content, and it is for me the best h-game out there.
This game started life as a Wincest title and only went to the 'Landlady' route after Patreon cracked down on that content from creators on their platform. So that's the game I want to play, and I mod it accordingly. It has what I think is pretty much the most believable story of all the Wincest titles I've played (meaning how MC comes to bed all the women in his family), and that goes a long way towards enjoyment.
But in a Wincest game, there are only so many possible options. Once you get them, that's it. That's the game; it would be a little disingenuous to suddenly introduce a grandmother or half-sister, or something like that at this stage (unless they could somehow be introduced earlier in the story). So anything else, meaning 'bangables' outside the family, is gravy. The teachers, Mrs Johnson, Mia's mom, Roxxy and her friends, Maria............. they're all basically just DLC as far as I'm concerned, so I don't care that updates come out once every 15 lunar cycles. It's nice to have all that content in the Summerville Universe, and I will gladly play any updates when (or if) they do come out, and that will be a fun way to spend half an hour that week, but that will be that. I don't care at all about the pregnancy stuff or dating, and I certainly don't care about sending MC to prom, or who killed his dad.