In the start of development, Hollow Knight: Silksong was originally intended to be a DLC to the original game, not an entire sequel. Silksong had since then become so large and content-packed that they had to change it into an independent game. Silksong was too hyped up to just be a DLC in the first place, however, and it should have been planned as a sequel to begin with.
Team Cherry realizes that Silksong is too large of a DLC to be a DLC. They just turn it into a sequel due to the large scope of the project.
Silksong was taking so long to develop to the point where there had been memes of Silksong release dates appearing, to the point where the community had almost given up on the game altogether. Though the reason that making the game took so long was that Team Cherry is a group of four people, and Silksong wasn't being made primarily for the people, it was being made as a passion project for them. They were having fun creating this game, and were having fun adding more and more content. This is also why it evolved from a DLC to a sequel. Though, during development, Team Cherry was mainly silent about the game, leading to this short of an article.
Team Cherry was having fun making Silksong, and didn't release too much information on development of the game. Content and story just kept piling on more and more until the game went from a DLC to the original game, Hollow Knight, to a sequel altogether. The gameplay makes the wait entirely worth it, and just shows how much heart and soul they put in to making this masterpiece of a game.