Formula and talent. For formula Alicesoft creates settings where sex happens instead of creating settings for sex to happen, this makes it easier for characters to have interests and personalities that interact with the setting. They also generally have a large cast, which means characters are less likely to wear out their welcome and the player can fill in any gaps with character scenes with his imagination.
For talent it takes effort to worldbuild the setting and correctly insert characters into it. More importantly, it's not a one man studio, meaning there's plenty of others to say "This sucks and here's why.". One man devs do get feedback from fans after release, but there is a significant difference between consumer and creator feedback.