Why do we use myths and elements from the real world?
For one thing, because they work. The myths and legends that exist in the real world have often been told and retold for centuries, or even millenia. And this means that they must have a powerful hold on the human psyche - for if they hadn't, they would have been forgotten long ago. So tapping into that font of myths is a chance of making settings, campaigns, and adventures more powerful - and what GM wouldn't leap at such a chance?
For another, because they are familiar. If you make up a totally original culture and history, you need to spend hundreds or even thousands of words to explain it to the GM - and what's worse, the GM has to explain it to his players. On the other hand, if you can hint at its similarities to a place from real-world history or myth, a few short words will suffice. It doesn't have to be exactly the same - but as long as the gaming group has some rough ideas about its real-world equivalent, and they all think on a similar wavelength, they can spend less time on wondering about the setting, and more on playing.
And IMO, that's why Tekumel always has and always will be a fringe setting. Sure, it might be very cool - but who wants to invest
that much time to understand the nature of a setting? Remember, not only the GM has to grasp the essence of a setting, but the players as well...
And incidentally, that's why I'm using real-world countries as a basis for many regions in
Urbis. Many elements of the setting are pretty wild, so it will help a lot if there are clear analogues to Switzerland, Italy, the Dutch lowlands, Arabia, Africa, Jewish ghettoes, the Industrial Revolution and so on...