I would venture to add one more reason, but one that does not have to be inevitable for the stores that remain. As an independent publisher in business 10 years, I can attest to the poor treatment I and my colleagues have received from some independent stores. Perhaps it's the perception that "local" can't possibly be good enough to endorse, so stores spend thousands to bring in a famous author from somewhere else for programs. This situation is doubly difficult for local publishers carrying quality literary titles by multicultural authors. While of course no store can take on every book prospect that comes through their doors, there are many award-winning authors from within that store's very own community (with impeccable credentials and a a strong public following) who get ignored.
It is my belief that a connection with the surrounding community is important, and that is directly tied to supporting talented authors within that community. This support also stabilizes the local economy. Perhaps independent stores will be more open to this idea as we move into the future.