Its the carbon. Without it you're oxygen ends up as oxides it the weld deposit as opposed to carbon monoxide and remnant carbon dioxide in the fume gas) which does not help the weld perform well under low temp impact testing.
That helps explain the Charpy's. The carbon also helps explain the the strength, as some of it will end up in the deposit as well.
But keep in mind, and this is often lost on many who agonize over strength differences with different gases, it doesn't really matter that much if your tensiles change to any great extent (given certain specific exceptions) if the min strength is still greater than the base metal. Even though your strength went from 85ksi to 70ksi, the 70ksi is still 10ksi stronger than carbon steel base metal mins.
So if you're gas gives you enough strength you look for other variables to optimize your application. Charpy's, cost, ductility, weldability, tensile to yield ratios, productivity, materials, etc. Its called engineering.