This is because it was Apple that devised the Agency Model and was the first to propose it to the Big Six publishers as a way of getting them on iOS. This led to Macmillan battling with Amazon one infamous weekend and had Amazon retaliate by removing all Buy buttons from Macmillan books. Amazon soon capitualted, other publishers -- except for Random -- piled on, moving to the Agency Model. When Random finally joined, all their books went up anywhere from $2 to over $7 in price.
"....which I think is understating the role of the publishers, who would have rejected it if it hadn't been very much in their own interests."
Apple just gave them a better deal than the one Amazon was willing to fight dirty to maintain.