This is true, and an obvious business decision. I assume if they offered more options (CPU, ram, etc) consumers would certainly buy them. I also assume that each permutation of the product will compound the logistics around manufacturing, marketing and so on.
The problem with Apple (well, a problem with Apple; they've more than one) is that if you need more then you are completely and totally out of luck.
So it becomes 'since not enough people need it, I can't have it.'