There could be a lot of different reasons. I'm guessing that the extra cost of manufacturing capacity as well as the extra cost of warehousing enough units would be the biggest problem, either on Apple's end or their suppliers.

Besides, wouldn't you love to operate a business where you could pretty much produce the product as it's needed: no worries if sales projections were overly optimistic, since you won't end up with surplus.

Don't get me wrong. Marketing definitely has a role here. For a lot of businesses, being unable to meet demand would be their death knell. For Apple, over supply would be a marketing disaster.