Question

why us diffusion insufficient ro meet the oxygen requirements of multicellular organisms like human

Answer 1

because in multicellular organisms only upper layer is in contact with the environment

Answer 2

In large multicellular organisms like humans oxygen of air will have to travel a distance inside the human body to reach each and every cell of the body .Now since diffusion is a very very slow process it will take a very long time to make the oxygen available to all the body cells .
Thus diffusion is insufficient to meet the oxygen requirement of multicellular organisms like humans because the volume of human body is so big that oxygen of air can not diffuse into the internal cells of the human body quickly so when the size of the multicellular organism is large than the respiratory pigment such as haemoglobin present in the blood take up the oxygen from air in the lungs and carry it quickly to all the body cells