Why don't human red blood cells have nuclei?

Mammalian erythrocytes are unique among the vertebrates as they are non- nucleated cells in their mature form. These cells have nuclei during early phases of erythropoiesis but extrude them during development as they mature in order to provide more space for haemoglobin. read more

I think the most important reason why red blood cells don't have nuclei or organelles is efficiency. Not only to make room for hemoglobin, but to not consume the oxygen they are supposed to be carrying to the tissues. The organelle system uses up a lot of oxygen. For a nucleus to function, you also need mitochondria. read more

A: Red blood cells have no nucleus, because most of their bulk is made up of hemoglobin, a compound that carries gases, such as oxygen and carbon dioxide. In fact, about a third of a red blood cell is dedicated to hemoglobin alone, so no room remains for a nucleus or many of the structures that other cells have. read more