Related to the question How long ago was the Dreaming?, according to the Wikipedia Rainbow Serpent page, the Rainbow Serpent is an integral part of many indigenous Australian cultures, and specifically
The most common motif in Rainbow Serpent stories is the Serpent as creator, with the Serpent often bringing life to an empty space
Why is the Serpent the most common motif as the creator in indigenous Australian cultures?
First, I took a look at Wikipedia. It explains that the Rainbow Serpent may have been inspired by one or more of the following Australian snakes:
These snakes could inspire cultures across the continent. Research tells me that the rough-scaled python is active in a small area of Western Australia, though its range may be much larger. The taipan lives in central Australia as well as the coast and regions beyond. File snakes live in aquatic areas across the region. An extinct snake, Wonambia, would also have lived in the plains; other large species existed. Given the geographic spread of these snakes, they all could have inspired cultures across Australia.
I independently found one of the Wikipedia sources, which supports the conclusions (of course!), as well as a related document. This is the only other non-National Geographic source I could find. The evidence, though, stacks up to give a sensible conclusion: Snakes were everywhere in Australia, so they provided a common habitat component. And the big ones sure seemed powerful.
One issue with this hypothesis is that some of the snakes have small geographic distributions - just look at the range of the rough-scaled python! There are other species of snakes in Australia, so they could make up the difference, but the limits of the three given snakes are a tad worrisome.
The snake as a symbol for creation is common across many cultures mythologies. The mythologist Joseph Campbell pointed out that
The serpent sheds its skin to be born again ... it lives by killing and eating itself, casting off death and being reborn, like the moon
Snakes have long been associated with fertility and the subtle creative force across many cultures. Their counterpart was often the bull who stood for active virility.
