In a livestream of Chapter 1 conducted for Undertale's sixth anniversary, Toby Fox stated that the chapter was originally meant to include a number of branching options like in Undertale, such as Kris being able to partner with either Susie or Ralsei depending on their actions up to the first K. Round fight. However, much of these ideas were cut both to avoid dragging out development and to streamline the plot; the resulting linearity ended up being incorporated into the game's narrative, resulting in Chapter 1's heavy focus on a lack of agency.
Contributed by game4brains