The game, which was formerly known as the "Sequel to The Legend of Zelda: Breath of the Wild", began life as DLC for the original Breath of the Wild. However, producer Eiji Aonuma elected to make it a full-fledged sequel, because there were too many ideas the team wanted to include:

"When we released the DLC for Breath of the Wild, we realized that this is a great way to add more elements to the same world. But when it comes down to technical things, DLC is pretty much data--you're adding data to a preexisting title. And so when we wanted to add bigger changes, DLC is not enough, and that’s why we thought maybe a sequel would be a good fit."
Contributed by raidramon0