Because of its unique plot, basically unlike other games where the focus is on the MC in terms of sex scenes, here the focus is on the girls. There are so many games focusing on the MC so those really have to be a lot better than the other games with the same focus to get a higher rating or even get a rating at all. Here what's happening is different and a bit rare.
I studied the ratings from this game and turns out it also appeals to a certain group of players mostly, who rarely get good games for them.
1. First I am talking about the players who like watching other girls being fucked by gross dudes while MC is sitting in the corner and isn't even allowed to jerk off to that visual. Ntr fans basically.
2. Bimbofication/corruption/female protagonist fans who just want to see some girls getting fucked up senseless but the objective here is different they aren't seeing it from the perspective of the MC, hell they wouldn't even want to notice that there is a MC in this game. They are just interested in how ans why the girls are doing the dirty things that they are doing, So it's not ntr or at least that's what they say.
So as I was saying these guys rarely get any good games with their unique tastes that's why whenever there is a good or even an average game with there preferred tags, they bombard that thread with absurdly high ratings.
If the same dev would have written a normal game where the focus was on the MC (no matter if it's as messed up as this game or as vanilla as MNF) his game wouldn't have got as much praise as he is getting for this one.
So if you are into any of these specific kinks you might wanna give it a try.