Probably because the women are Sexy, and it appeals to a nice round of kinks, has good voice work, SFX, and solid animations, ergo "good fapping material". The Simulation aspect as basic as it's written, does also present itself as a mystery so i can see this adding a unique factor to it. Popular won't necessarily equate to good, for every BaD there's a Summertime Saga. I also Imagine that INCETON knows what their fanbase likes, so their able to continuously appeal to them(the fan-driven-writing process helps a lot). In essense INCETONS fans are probably ppl looking for fap material, as opposed to an interesting experience(nothing wrong with that, sometimes you just want to rub one out).
the Steam version, adds dialogue to Liz's(MC's Mother) scenes so she's pretending to be asleep when he's raping her, a sex scene at the beggining with a "foreign student", a full on title scene, Blackmail options for Nicole, and more SFX to the sex scenes to make them more immersive.
I would say go for the Steam one.