Uni cellular are the single celled organisms,they live on simpler form of food(organic food)they just engulf the food in its food vacuole and after cyclosis it excretes it out,it gives the organism enough energy to live,uni cellular organisms have their body made of water or can say it is a jelly like structure(like in amoeba).so they don't need the food to make blood and run through the vessels,that is why they donot have circulatory system as well as other systems like in multi celluler organism.
The structures of a circulatory system are made up of cells which exists to move materials the organism needs to live. When the organism is made up of only one cell, all it needs is inside and outside that one cell, and there is no need to move this material anywhere else.
Because they're only made up of one cell so there's no need for a circulatory system because they don't need to travel to places inside a single cell.
Actually, unicellular organisms do have systems for moving nutrients into the cell, moving intermediate products from one part of the cell to another, and moving waste products out of the cell.
It should be self explanatory to you as having only one cell ,then there is no need for a circulatory system .