But Slytherin didn't create Voldemort; he was a product of lacking a soul, his ambition to conquer - you could say, his ironclad determination to hurt others like he had been shunned, outcast and institutionalized since birth. All evil, no matter how strong it is, has an origin - and no one can know about his past because it died with him.
We have to look at the unity of houses when students grow up; most commonly it's going to be with their peers from Hogwarts. The Potters are well-aligned with the Weasleys, the Longbottom(s), the Tonks; except for the Lovegoods. How much diversity exists within the family's immediate social circle unless one of the other children is sorted into another house?
History hasn't taught the post second wizarding war children anything different. The next generation knows about Harry versus Voldemort only if they talk about it with their parents, or by what society shows them. So what is Harry signficantly telling Albus when he says you can choose Gryffindor if it will make you happy, peaceful, content with who you are. Not Slytherin housed one of the bravest men that he knew (this is up for debate with your own views of Severus) and leave it that. Unless Hermione is inspiring her children to go on a House Equality rampage, most of society is still breeding the same anti-this, pro-that messaging.