Why is chess haram as it is not a game of chance?

We do not know the reason for many divine orders, but we follow them on the base that they are issued by God Almighty. There are narrations in chess that it is forbidden, and therefore the scholars said so.
Yes, there is a difference in fatwas. S. Sistani says: If chess turns into a mental game, man learns to think, and no longer a game of gambling, it is possible to play it.