There's another term which might explain the phenomena as well--one closely in line with the Zeitgeist explanation. In 2002 Stuart Kaufmann introduced the term "adjacent possible" to describe the space of ideas that we can reach, from where we are. He used it in reference to biology but it explains human creativity well, too.

Basically, each new step we take forward in our technology is dependent on what we have discovered before. Photography itself depended on several inventions that were first required (say, perhaps, the camera obscura and a couple chemistry-based light-capture techniques) before it could even reasonably be thought of.

Once those pre-conditions were in place, it was only natural that at least a couple creative minds would connect the dots, so to speak, and take steps to make the new possibility (photography) happen.

It's one of the best explanations I can think of, and gives a more technical explanation of why some ideas are "ready" to be brought into the world. New ideas are built from connecting old ideas -- once the old ideas are in place, multiple people will start drawing the connection, because it's just right there, waiting to be drawn. The adjacent possible.

J