Supermarkets and full-service dept stores (like Macy's) are in decline. What factors might these stores have in common behind their decline?
The most likely factor that these have in common is that they try to be many different things in a time when specialization is becoming more and more the norm in American retail.
Thanks to the internet and the specialized stores that it makes possible, more generalized stores are having a hard time remaining in business. They do not specialize in any one thing and so they tend to be a little more mediocre in many things. This is the classic idea of being a "jack of all trades and master of none."
Another possible factor is higher costs. Supermarkets and full-service department stores both try to have better customer service and higher quality goods than discount stores. But these stores then get caught between the high end speciality stores and the discounters. They are an in between sort of store that does not fit in either niche and loses customers to both.