While Tesla may be working on many new supercharger locations, it can take longer than they expect to secure a specific location and get the needed approvals to get the power they need and build the supercharger.
I believe some of the future locations on the 2018 map may have been on the 2016 map - we're still waiting for the last superchargers to be added for the east-west route in west Texas.
Tesla's working on expanding the network - but they don't control all of the timing, when they are depending on securing locations, government approvals, or getting a construction team.