Why do red blood cell eject their nucleus when mature?

Why do red blood cell eject their nucleus when mature?
so they have lot of space to pass oxygen.

Erythrocytes in mammals are anucleate when ripened, meaning that they absence a cell nucleus and thus have no DNA. In comparison, the erythrocytes of nearly adjectives other vertebrates have nucleus; the only set exception is salamanders of the Batrachoseps genus.[1]} Mammalian erythrocytes also lose their other organelles including their mitochondria and produce energy by fermentation, via glycolysis of glucose followed by lactic acerbic production. Furthermore, red cells do not hold an insulin receptor and thus glucose uptake is not regulated by insulin. As a result of the lack of nucleus and organelles, the cell cannot produce new structural or repair proteins or enzymes and their lifespan is fixed.
Mammalian erythrocytes are biconcave disks: flattened and depressed in the center, beside a dumbbell-shaped cross section. This shape (as powerfully as the loss of organelles and nucleus) optimizes the cell for the exchange of oxygen with its surroundings
cellular division
the concave shape increases their surface nouns, which works great while absorbing oxygen.