It was just too windy at Cape Canaveral. SpaceX fueled anyway, just in case, but the winds never settled down and they are now draining the tanks. Tomorrow the forecast is better for their second attempt to land the first stage.
But SpaceX will still be landing something today: the CRS-5 Dragon separated from the ISS this morning, and will splash down west of California this afternoon (Pacific time). Here’s the unberthing video: