The space shuttle can't launch through thick clouds. A build up of excess moisture weighs the ship down too much. It also interferes with recovering the boosters that drop in the Atlantic Ocean. Closer to launch, the sky was fairly clear over the shuttle, but this cloud and its friends were all around elsewhere, so it would have interfered with all the other things that have to fly around after the shuttle launches. I suspect they also want a clear sky so they can get better video if the vehicle tries to copy-cat the Challenger, too.