Why is “The steerage” hailed “one of the greatest photographs of all time”?

From https://en.wikipedia.org/wiki/The_Steerage:

The Steerage is a photograph taken by Alfred Stieglitz in 1907. It has been hailed as one of the greatest photographs of all time because it captures in a single image both a formative document of its time and one of the first works of artistic modernism.

The Steerage

I just don’t get it. It is a document for sure, but why is it so important? I don’t have art background so I’m not sure what “artistic modernism” even is (“artistic modernism” image search yields pictures that are very different in style).

I can see how it’s a reasonably good photograph, but I have no clue why do people say it’s “one of the greatest photographs of all time”. I also feel the same when seeing photographs in art galleries. What’s the deal? What am I missing?