I think the amount of content also plays an important role. A game with multiple routes or alot of dialogue and scenes is more replayable than a brief plot with nearly no scenes. Though for that, the graphics and story should be at least decent.
These days, I think uniqueness in art style stands above all. That's why Summertime is big and inspired a few imitators despite every other aspect of the game being pretty basic (to say nothing of the update schedule). Similarly, the Light of my Life characters are not conventionally attractive but they are unique and have personality.
If we are talking about earlier times, I think it's just good timing and a healthy dose of luck. When you are early to a thing, you get more benefits from the thing - that's a universal principle. Amazon did not start with a master plan. They got into online delivery services pretty early and they got pretty lucky to stand out among competition.
Graphics are the most important part of games. The story can be the most banal of all or a completely unique reading experience, and the characters can be mindless embodiments of tropes or more lifelike than life, BUT if the graphics are a poorly crafted creation on HoneySelect, DAZ, Koikatsu Party - it's unplayable and unfappable. This is all compounded by the fact that the vast majority of games are created using these tools. So when someone at least tries to draw something their own way or uses the aforementioned tools very skillfully, I forgive any flaws in the plot, characters, technical aspects, etc.