Apple themselves said today (based on mr story) that apple has asked more gold phones than originally order to the manufacturers to try to satisfy the demand that they didn't realize existed for gold.
That was one of my options. That they mis-judged demand doesn't mean they intentionally limited demand. If anything, i'd bet the thought process was "the last time we did gold it didn't sell well. Our data says gold is more popular now, but the last thing we want is a device people wont "settle" for because it's too garish. So let's go with what makes the most sense, build what's sold well for us so far, and we can always change up later if our predictions are wrong. At a minimum, we won't have a ******** of Gold phones people just arent buying.
Remember, the exec is tim cook. He's the consummate supply chain thinker.