I am trying to buy January 2018 NG future in commodity but zerodha does not allow. Whereas other brokers do.
Some brokers do and some brokers don’t. The liquidity in these far month contracts is patchy and can dry up any moment. And there have been cases where people haven’t been able to square off their positions resulting in exchange penalties. So the RMS teams of brokers block of contracts based on the risk they perceive to be.
As for your point about other brokers, just because they do it doesn’t mean everybody has to do it.
For non-agri commodities, only current and next month contracts are available for trading and for agri commodities, only current month contracts are available for trading.
I assume this is done keeping the traders interest in mind. There were certain instances in the past where the spread in far-month contracts was large and those who probably placed market orders always ended up buying higher or selling lower. Also, thinking from the risk point of view, if there is no liquidity and there is a margin shortfall in your commodity account, then this becomes a hassle for the risk team to square off your position as well. They might have to carry your position to the following day for which you might incur a margin penalty.
Current and next month has good liquidity. You could certainly trade them.