Because for a manufacturer this long in the business, there's absolutely no excuse for not meeting demand. Granted, they tend to use proprietary parts so it's entirely possible their system designs create a shortage of parts leading to supply issues. Honestly though, they prefer to have 100% sell-through of inventory so they limit production not only as a cost savings on assembly lines, but it looks great in a sales meeting. Who wants to be the person explaining cratering sales figures to shareholders?
If you don't know the demand how are you supposed to determine the supply? Saying that it is purposefully done is silly. They said themselves demand for the NES Classic took them by surprise.