"still almost 100% techies in India never prefer or think about joining ISRO/DRDO which are world-class research organizations."
I've consulted for both the DRDO and the ISRO on some very technical projects.
You are partially right. Manpower is a problem in both organizations, with the best people going to the United States. That said "world-class research organizations" is a bit of a stretch. There are specific departments which do world class research, but by and large ISRO and DRDO are far from that ideal.
Plenty of "scientists" pushing paper and just wasting time in the office and waiting for 5 o clock in the evening so they can go home. Being part of the Indian government, the wastage of resources is appalling (to someone who's worked in the private sector / run his own business)