Well, first up: even at their peak, the concentration of CFCs in the atmosphere barely cracked the parts per *billion* level - so that right there is a good example of how concentration alone is not a good measure of effect.
Now, why CO2? Well, firstly keep in mind that *all* climate scientists also take methane into account - so that's a bit of a *** you've got going there. So why CO2, methane and water vapour (and, for that matter, CFCs)? Because unlike the nitrogen and oxygen that make up the vast bulk of the atmosphere, these are the gases that absorb infrared.
The only reason I can see that people find this so mysterious is that we can't *see* infrared. To get a better feel for the problem, try this simple experiment at home. Find a nice big, clear glass jar, and put a litre of water in it. Put two drops of blue food colouring in (this will give you a few hundred parts per million). For extra credit, put two jars of water (one with the food colouring, one without) in front of a nice powerful light source, then come back in a few hours and measure the temperature of each.
