Why did the confederate soldiers not pursue the union soldiers at the battle of bull run?

The reason that they did this was because they truly didn't have much of a choice. Back then, soldiers had a different formation that we see now like in CoD or Battlefield. and even though they didn't know how many soldiers were down there, they didnt want to risk it. they also were not sure if it was worth it to charge the soldiers because of the fact that there wasn't much to salvage down in their camp if they won. 

hope this helps :)