I don't think Apple had ever released a budget device together with their premium offering. I suspect their strategy has always been to release the high-end models first, let the market buy its fill, then release the cheaper option early next year to avoid cannibalising sales. It's also a good way to spread out demand over the year so you don't have too much unused capacity.AAPL (clearly) also benefited from the April 24th release of the 2nd-Gen SE, which was probably planned-for in late-2019 !
It's probably easier to predict demand this way - how well or poorly your sales do will give a pretty good picture of how much or little to target the following year.