Because these smaller cells can access and pass through the membranes of the other cell membranes and permeable coverings easily and voluntarily unlike larger cells. It is also for efficient for these smaller and miniscule cells to penetrate and travel, delivering and transporting goods and materials such as nutrients, oxygen and waste throughout the body without consuming enough energy and conserving lesser space.
For simple reasons, they are faster, more efficient and consumes little space, and most especially depletes lesser energy.