From experience and reading lots of dressage training books and websites/articles. It is really tough for a horse to learn to go sideways, it is nearly impossible to take a horse out of the feild and ask it to go sideways while bending, so we have to gradually introduce these concepts, allow the horse to understand the aids and build the necessary muscle.
By starting with the leg yeild, we teach the horse to go sideways. By doing counter changes in the leg yeild, we ensure the horse is not falling through any aid and understands the purpose and means of going sideways. Then the shoulder-fore teaches the horse to stay in a bend while taking sideways steps, then we increase the angle as the horse gains understanding and muscle to end up in a shoulder in and from here we can introduce the travers. Although the horse has the same shape in the shoulder in and travers, just in a different orientation, it is beneficial to teach the shoulder in first because it is easier to maintain control over the shoulders and teach the horse about the outside aid.