Because of the Princess Maker 2 "kiddie porn" effect (better to Google it for the particulars). The PM2 that's in Steam HAD to remove that "feature" in order to sell in the US, and even then, Gainax had a problem licensing it as they got diplomatic notice the last time. There are certain things that do get censored for legal reasons on their way to the US.
Editing for legal reasons is fine with me. There's no need to have questionable material such that the US can beat your head over it. US v Whorley (2008) is the current law.