It's... a number of things. Each one is perhaps not a deal-killer by itself, but when added together, and when you realize that trying to change any of them would ruin the game completely, you understand the issues that would be involved in trying to bring the game over. First and probably most obvious are the "SGs", and the related "punish" sequences. Yes, these involve a lot of nude imagery. These are, amazingly, the smallest problem they have. It is very easy to misinterpret the "punish" sequences at the end of most chapters as a kind of voyeuristic tearing-down. They aren't, though; they're about confronting a character with their problems and making them realize their issues. Perhaps not accept them at once, but it's ultimately an attempt to help. There is probably a bit of tragic (in a meta sense) use of arousing imagery to get the otaku cadre in Japan to care about the game, but these sequences are ultimately not about the visuals at all, and the game makes the jarring disconnect work for it. But all a lot of critics will ever see are revealing images and the player character being "cruel" to other characters. (The images themselves, of course, are a ratings-board minefield, but that's a separate issue.) Second is the nature of the antagonists... and you'll forgive me if I'm very vague here, as I don't want to spoil anyone on this, even in a spoiler box. The antagonists, needless to say, fit the game's advertised theme of "eros" very well. Exactly well, in fact. To the point that they serve in some fashion as a counterpoint to antagonists in Fate/SN and the original Extra. Their arcs ultimately offer up a number of ruminations on the nature of attraction, on the nature of desire, and yes, the nature of love, in the romantic sense and other senses. It's very obvious Nasu wanted to ruminate on a lot of things in the narrative of this game - and yes, the game is written by the man himself. But. Due to the way they are portrayed in parts of the game, I am absolutely certain that may critics would - willingly or otherwise - interpret the antagonists as a broad "attack" on women. That the game's message is that women are to be feared and fought. Which, if you're paying an iota of attention, isn't the point at all and is nearly the opposite of the point - but I can already see the arguments in my mind's eye, and they are ugly and damaging to the company that dares to attempt to sell CCC. Finally is the game's violence. Ironically, this is more a problem for Europe than it would be for America, since America is completely inured to violence at this point, but some sections of the game are disturbingly violent. Ironically, it isn't always graphic violence or isn't shown in the worst way it could be, but it's there and sometimes it can be very unsettling. Let me put it this way: at one point a character is impaled through the chest (and this is quite graphic), and another time a character is torn in half by an assailant. These events are common to all routes in the game. It's not quite Fate/Zero levels of brutality, but it gets close at times. The short of it is, there's all kinds of issues that make the game a poor fit for export - and many of these are, ironically, the things that make the game so great. To try and change them would deface the game... but it's easy to see how the modern criticism apparatus, and even modern American (in particular) culture, would badly misinterpret the game, often without bothering to finish it. There's simply no way it would be worth the heartache and bad press, especially given how much work would have to go into the localization effort.