I decision to go with only 64GB and 256GB variants is a pure margin play. By creating such a large gap in capacity, Apple knows that many will chose to go with the larger capacity variant and pay hundreds of dollars more for something that on the BOM likely costs less than $10. At the same time, the BOM cost difference between 256GB and 512GB is probably a larger spread than 64GB to 256GB, hence the cap at 256GB. Long story short, they’re playing us for our money by creating an illusion of choice when in reality we have none.