I think this highly subjective as most historians can’t agree on what he was trying to say and why. But my interpretation was that he used the term “iron curtain” because iron is viewed as strong and impenetrable. Remember, the Soviet gov controlled their people. The news they got, the tv they watched, the flowing of information in and out of the country. Nothing from the outside was getting in unless the gov approved it. Also in the flip side, the gov wanted to keep its people in the country and they could not escape easily. This is why the Brandenburg Gate, The Berlin Wall and yes the physical Iron Curtain were constructed.