Elite ( except a few) is not interested in CSS because THEY DONT WANT TO WORK FOR SUCH MEAGRE SALARIES.
If u see the list of top 40 candidates every year, FSP is the no 1 choice of Elite or Upper Middle class who have more exposure of the world, so, they smartly choose FSP due to its perks and good salary.
Majority of the lower middle class people choose PAS since experience colonial mentality of our society and because they have very little idea of the exposure FSP provides.