Or that they just didn't order any more. Amazon can still track how well, or not, the Mac products are selling through the 3rd parties that sell on their site.
If it isn't high enough volume and/or margins aren't there is no good reason why Amazon has to stock in their warehouse.
Except that the base Mini is their 10th best selling desktop computer as of right now (and don't forget it's been out of stock for months) and is their 2nd best rated in the same category.