It's not as easy as clicking the "Add country X"-button, there is work involved with each addition. And the gain of doing that work just to add bland and unfamous civilizations with the sole aim of selling a few extra copies in said civs current country/countries, is questionable. And I say a few extra copies, as I don't think the number of people who buy games solely for their country being mentioned is that great.
There are many mods which can just be adopted and used in final version. Final effort would not be so big.
The addition of new CIVs was one of most importent changes for BTS, why Fraxis added 10 contries if not because of those "few" additional copies which would be sold in added countries (of course not for all, some not exist now :] ).
Giving more variety of civilizations in possible games is also a good point for all.