Yes, there is bundle oflogic reasons behind it..
1. PCS is the service which has a definite infrastucture in terms of their powers, perks n previliges (houses,vehicles,servants), staff including force etc. It means when u r going to join u will get the desired facilities. warna to Bs-18, 19 man to har cheez har group man mil hi jati ha.
2. Baqi rahi IRS ki baat they dont have definite infrastructure but they will start to make it now. It means u will strive for that from now and ur juniors will definitely prefer tis service.
3. So at this level, PCS should be preffered over IRS.
tc