This happened quite some time ago. Unless you're in the Southwest bean counting department, I don't know if any of us will over know the exact details or when and if they'll provide service again.
What probably is the case is that Southwest has had a rather static inventory of aircraft when compared to the new markets they've entered and want to expand service to. Essentially, they could make more money with the aircraft they have in markets other than Key West. Same basic economic decision as cutting service to Jackson, MS.