Well, B&W film has always been much easier to develop and was mostly what was used in media print up until about 20 years ago. Digital really didn't start becoming a popular medium to shoot with until about '95-'00 and up until about 2000 it was extremely expensive and didn't provide the quality that film did. So you have about 20 years of photos that will mostly be color vs. pretty much everything from before that to choose from. I mean, there are exceptions like the one posted below, but normally when people choose an image they believe is iconic it's not something they'd pull from Rolling Stone.
One of the last photos taken of John Lennon before he was assassinated. Nopefully it's not too NSFW for the forum.
From one of my favorite films, "City Lights". This image is at the very end and it's emotion is easy to understand when you know the reason behind it. Although it's certainly a comedy, it's also one of the greatest love stories. The recipient of Chaplin's love and sacrifices realizes what he went through so she could regain her sight. What seemed to be a case of unrequited love was finally realized and Chaplin's raw emotions made for one of the great iconic images in 20th century entertainment.