Why does this compound only have one pair of enantiomers?
Usually it doesn't as rotation around single bonds is generally very quick in room temperature. However, sometimes it matters because rotation can be blocked by bulky substituents, or other parts of the molecule. Even double bonds usually connected with E/Z isomerism, can in certain situations cause optical activity.

These are all examples of so called axial chirality. In the first case (rotation blocked by substituents) it's called atropoisomerism. Second (blocking by other parts of molecule) is present for example in helicenes. Third case (cumulated double bonds) occurs in allens.