This game doesn't need a complicated story. Being a DIK is a college sitcom, kind of like The Big Bang Theory or How I Met Your Mother. They have mostly localized plots, and there's almost no overarching plot.
Besides, if you think back to the teen comedies about college in the 90s, was there any plot there? The plot of American Pie is to lose your virginity before you graduate from college. And the whole comedy is about a bunch of losers trying to do that, and it creates comic situations because of it.
The plot of this game is in the title, What It Means to Being A DIK. Being A DIK is both being a fraternity member and being a party guy.
Overall, the lewd scenes are so good not just because they are well done. The characters don't look like fuck dolls like in games like Inceton Games or Genesis Order, but like living people and so every lewd scene is an emotionally evocative event. The main reason for the popularity is the characters, not the plot.