The company explained with more detail on its site:
Unfortunately upper-level winds continue to exceed acceptable limits and are expected to get worse as we approach tonight’s launch window, so we are forgoing today’s launch attempt. Winds are forecast to exceed acceptable limits through Thursday. Our team will continue working with the Air Force’s Launch Weather Officer to evaluate the best available opportunity for flight in the coming days.
This would have been the fourth attempt to launch the SES-9 satellite into orbit--a mission that has been long delayed due to the June 2015 explosion that left SpaceX's Falcon 9 rocket grounded for several months.
Although SpaceX has been insisting that the rocket and spacecraft are perfectly fine, the company has been particularly finicky about the launch conditions. Because of the delay in launching the satellite, the Falcon 9 rocket will carry the satellite higher up, getting it closer to its destination to make up for lost time, so the company will need every drop of fuel it can spare to attempt landing the rocket booster on a barge ship after the launch.