Question

why there is no mitochondria in erythrocytes?

Answer 1

The reason why mature red blood cells (erythrocytes) have no mitochondria, is that (most) RBC organelles (including the nucleus) are expelled during maturation. RBCs need very little energy for what they do — shuttling oxygen from lungs (or gills, or trachea) to tissues, and shuttling (some) carbon dioxide[1][1] (and other waste products) from tissues to lungs — RBC energy (ATP) requirements are satisfied by glycolysis, and by fermentation of the resulting pyruvate.

RBCs stem from bone marrow, and once mature have a limited “lifetime” (some 120 or so days), during which their “main function” is to transport gases. By getting rid of (most) organelles, space for more hemoglobin is freed up, which is what the inhaled oxygen binds to.