The Hanafi position discussed here requires the Salam commodity to remain available in the market from the contract date until delivery. The concern is practical certainty: the seller should not promise something that disappears from ordinary supply during the contract period.
The Shafi‘i, Maliki and Hanbali positions described do not require availability on the contract date; what matters is that the commodity be available when delivery becomes due. The latter view is presented as adoptable in modern circumstances.