Unlike multicelled organisms, a single celled organism does not have a circulatory system. Which is the reason for this?

This is because they don't need a circulatory system. They don't have things to be transported around.

Answer - One of the few reason why a single cell organisms does not have a circulatory system.

They only have one cell which means they do not require specialize system to get materials moving around. In turn allowing the cell to get materials even more easily then multi celled.

Reasoning - Since Single celled lack system it can easily gain materials fairly easily then multi celled which goes through processes.