I have taken this information from the "Praxair shielding gases selection manual". The reason Argon isn't used as a stand alone GMAW gas is due to the phenomenon of surface tension.
"In any liquid there is an attractive force exerted by the molecules below the surface upon those at the surface. An inward pull, or internal pressure is created, which tends to restrain the liquid from flowing. Its strength varies with the chemical nature of the liquid.
In welding, the surface tension between molten steel and its surrounding atmosphere has a pronounced influence on bead shape. If surface tension is high, a convex, irregular bead will result. Lower values promote flatter beads with minimum succeptibility for undercutting.
Pure ARGON shielding when used with GMAW is usually associated with high interfacial energy, producing a sluggish puddle and a high-crowned bead when mild steel welding is considered. This is partially attributed to the high surface tension of liquid iron in an inert atmosphere. For this reason, it is not recommended for use in MIG welding of mild steel. Iron oxides, however, have a considerably lower surface tension and thus promote good wetting to the parent metal. Therefore, the addition of small percentages of oxygen or carbon dioxide to argon when performing GMAW result in a more fluid weld puddle."
This basically explains that steel needs the oxygen from CO2 or straight oxygen to form iron oxides during the welding process for puddle fluidity.
hope this helps.
CO2 is used to weld carbon steel (Mig and Tig) for one reason and one reason only..........cost......I will take a bottle of argon anytime, as long as you are buying it. CO2 is used on Carbon steel because iron oxide melts below the melting temperature of the parent material and because it is cheap. A little O2 does help with puddle fluidity, but straight argon=no splatter so there is a savings in cleanup. And straight argon will have a deeper penetrating bead which is an advantage on anything .250 and thicker.