The rationale behind having a pre-market session was price discovery and stable market opening. Markets opening at times used to be very volatile and took long time to find a proper direction. Hence initially exchanges started with trading in the 50 stocks of NIFTY50 & 30 of SENSEX - so that the Index could open stable. Last year they have included other scrips too.
Now why only Equity is because as I mentioned the idea is price discovery and its the underlying which dictates the price and not the derivative. Hence only Equities are allowed.
Nice one Rakesh…