No one would know how many units they would sell. It is like Apple and Samsung announce their new products right before launch to minimize the reduction in sale of their existing line.
If DJI produces too many units, there would be a high overhead cost with tons of Phantom sitting in storage. Price would drop due to over supply, and distributors would more eager to offer discounts to help facilitate sales. Ultimately, this would cut into the profit margin for both DJI and the distributors.
If DIJ produces "limited" amount of units, they would create a supply shortage. This can help distributors sell faster and keep the price higher for longer period of time.