Looks like some were rejected because they promoted five core skills students should develop: self-awareness, self-management, responsible decision-making, social awareness, and relationship building.
I guess they just want math books to be about math.
It's weird my comment here is being down-voted when its pretty much a quote taken from the article. I put it up as a TLDR so people who were interested could read more, and those who weren't keep skip past.
Downvoting is childish and is a bad feature to have, being inhererently opposed to the ethos of free speech. Most downvoting is done by progressives on HN, but it seems likely that one of my fellow conservatives got you.