Transport of carbon monoxide (CO) is diffusion limited because
Since CO binds to Hb very rapidly, there is always a pressure gradient for its diffusion. 
There is no free CO in blood. CO diffusion can be increased only if the thickness of the capillary decreases or if the area of the capillary increases. 
So transport of CO is diffusion limited.
High affinity of CO for haemoglobin
Alveolar membrane is less permeable to CO
CO crosses epithelial barrier slowly
On exposure to air there is sudden increase in partial pressure