I have a feeling part of it is many people who create fad crosses don't really have an understanding of genetics so they assume if you get all black chicks from the cross you will never get white chicks. Or they just have Leghorns on hand and never even consider getting a recessive white breed. To be fair Leghorns are the most common and more similar in type to Cemanis but I believe white Phoenix would be better. And Cackle hatchery has those!