The article addresses that, albeit very shortly: "It may seem arbitrary to focus on X, one of 23 chromosomes, but it has 8 percent (1,529) of all our genes, including some for fertility and intelligence, which affect reproductive success."

Besides, it's one of the easiest things to see, and hence one of the easiest to look at - and, as it's sex-linked, there is a very clear way to see its effects (differentiating between paternal and maternal grandmothers, and their effects on their grandchildren of either sex). All the other ones have exactly a 25% chance to be transmitted to grandchildren, regardless of the path, and are therefor even more useless to look at.

I must agree, though, that it would be nice if the article had included more statistics about grandparents effects on the survival of children, and less explaining their theories and math.