Why is diffusion insufficient to meet the oxygen requirements of multicellular organisms like humans?

Diffusion is a very slow process, and can not be used to meet the requirements like oxygen uptake by humans. Diffusion is effective in smaller unicellular organisms which require lesser energy and are in direct contact with the environment. 
Multicellular organisms like humans have an array of requirements, for which specialized cells perform different functions of obtaining oxygen, food, etc. 

At two points close together toward the upper right of the diagram, since X and Y chromosomes are more advanced.