Why didn't the US sell Germany helium for Hindenburg?

The United States had imposed a military embargo on helium against Germany.

However:

The gas used for lift was not the main culprit of the Hindenburg accident. The outer skin had been doped in compounds to water proof and reflect light. Unknown at the time was the combination of the two doping compounds was a main ingredient in solid rocket fuels.

Helium would not have prevented the Hindenburg accident.

Documents later uncovered show that the German government discovered their mistake and quickly covered it up. This action has led many people to the false understanding that the hydrogen lift gas caused the accident.