Why are 99.99% of Af’s Paris based Int’l flights out of CDG, but its domestics out of ORLY??? 
8) 
First off, I am in NO WAY an expert on this subject, but I have a rather simple idea: Think NYC. Until fairly recently (with much thanks to jetBlue and others), KJFK was seen as a strictly international airport with very few domestic routes (most of which were basically connections for the int’ls.) LGA and EWR were used more for domestic travel, simply because there was enough of a demand to support all 3 airports in this setup. Now, KJFK has more domestic service than before, and KEWR is serving BOTH domestic and international routes. KLGA is still mainly used for domestics, especially short-medium range flights from the east or midwest.
Shorter Answer: BECAUSE THEY CAN. It makes sense for an airline to utilize different airports for different purposes, and this appears to be what Air France is doing in Paris. I would imagine the mechanical costs would be reduced somewhat, with certain airframes flying out of 1 airport and not the other, allowing mechanics to “focus” on the birds that they see the most of. From a traveller’s view, it helps to know for sure which airport you’re going to be flying into, especially for the folks who have to GET the clueless tourists to/from the airport. I would venture to say that well over 75% of all travellers pay no attention to airport codes, and most probably don’t even realize there are 2 airports in Paris, so there could be some confusion if Trans-Atlantic flights flew w/ similar frequencies from both. Does this make sense? I realize that this contradicts how London (BA in particular) utilizes KLHR & KLGW.
I could be wrong; don’t be surprised if someone chimes in and tells me I’m an idiot. I hope this helped a little. 
Anther thing I didn’t think of (it’s late here):
Perhaps ORLY doesn’t have the capability regarding Customs, etc. to handle internationals as well as CDG. This would be another added cost to keep Customs offices and staff piled up at both locations, when 1 port may have enough capability on its own. The new motto for the airline industry is “Simplify!” This goes hand-in-hand with cost cutting, which I need not remind you how important that is in this day & age.
Regards!
The French government has placed a limit on flights to Orly that limits capacity even though the airport is capable of handling much more traffic than it does currently.
The lack of flights using Orly is the result of politics.