Because Deep Work is more a "how", while The Shallows is more of a "why". It's good to know why, before you try to get to know how. Specifically, The Shallows is an investigation into what the Internet and modern technology does to the brain and what kind of side-effects it may have (affecting our ability to focus). It's a very well-written and accessible book.
Deep Work is more of a how-to book - about how to get ahead in world that's losing its ability to focus (due to the stuff described in The Shallows).
Deep Work is more of a how-to book - about how to get ahead in world that's losing its ability to focus (due to the stuff described in The Shallows).