Why is the dark reaction called a cycle?

The dark reaction is considered as a cycle as the formation of RuBP at the time of the reaction is used to begin the reaction again.

Explanation:

In dark reactions, the organic energy molecules formed at the time of light reactions, that is, the ATP and NADPH are used, and the reaction cycle takes place in the stroma. The reaction cycle is also known as the Calvin Benson Cycle.  

The dark reactions begin with the carboxylation of RuBP or ribulose-1,5-bisphosphate in the existence of an enzyme Rubisco. The phase of the regeneration of dark reactions produces the component RuBP again that then moves into the next phase of the dark reactions. As the initial component is reproduced again at the time of dark reactions, there is a formation of the cyclic path taking place.