Diffusion is comparatively a slow process when the metabolism of breathing is taken into consideration. The transfer and diffusion of gases occur within a fraction of seconds. Also sometimes at the cellular level, gases have to be exchanged against the concentration gradient where Diffusion cannot occur.

Plus multicellular organisms have specific locations defined for the Diffusion, those location (alveoli in humans) are not in direct contact with environment hence, Diffusion can not meet oxygen requirements of multicellular organisms.