This is not confirmed by SpaceX, but has been floating around the internet as the suspected reason. Last year, SpaceX attempted a first stage water landing, bu

This is not confirmed by SpaceX, but has been floating around the internet as the suspected reason. Last year, SpaceX attempted a first stage water landing, but was not successful. Their two most recent successes on this front had at least two hardware differences from that old attempt: landing legs and heavier duty RCS thrusters. However, due to some combination of manufacturing and launch scheduling, an old core without these improvements, similar to their attempt last year, will be flying on flight 13.