There is proper practicle reason.
There r arbitrage funds, who trades with 0 risk.
They used to buy shares in cash and sell equal amount of shares in futures.
At the time of expiry there profit will (fut price- cash price).
Irrespective with any direction market may go, there profit is locked.
If fii buys futures implies gap between fut and cash increase which gives opportunity to these arbitrage funds.
Who can buy in cash and sell in future.
By that way in cordination cash price can be adjustable via derivatives.