True. One reason for that is sporadic liquidity of these instruments on NSE/BSE.
Though a volume of hundreds of crores of INR exists across GSECs in these secondary markets, it is spread across 100+ such bonds, with many days having even a lakh of transaction volume (if any).
Plus the other pet theory i have is that folks sell-off GSECs (and other such relatively low-risk-low-return assets) at significant discount, for immediate liquidity during market downturn to invest in potential higher returning assets in the immediate short-term.
However, this is simply a guess based on correlation between GSECs/SDLs/TBILLs being available at significant discounts on their face-value during periods of significant equity market downturn.