Is it because CO2 is more soluble in water or has it got to do with the alkaline pH of plasma ?I here mean only aqueous CO2 transport from body cells to lungs.
The bulk of CO2 is transported as bicarbonate/carbonic acid. The conversion between CO2 and carbonic acid is catalyzed by carbonic anhydrase in red blood cells, otherwise the conversion would be very slow. The bicarbonate is then shuttled out of the RBCs and into the blood plasma. The CO2 (gas) in blood plasma accounts for <10% of CO2 transport, and the solubility isn't that much different from that in water.
Source: Guyton and Hall Textbook on Medical Physiology