This is most likely not a historical thing, but one of conveience.

*If you have just two limiting dimensions, like the height and width of a wallet, a rectangle will fill that space with the most surface.

*Long edges are easier to handle. Ever try playing with one of those custom decks of cards shaped like circles or dog bones or footballs?

*Writing systems in most times and places have been orthoganal and so the best thing to write on is a rectangle. Latin, Cyrllic, Runic, Egyptian Heirogylphs, Arabic, Mongolian, Chinese, Korean, Japanese, Urdu, Mayan- they're all written in parallel straight lines, so rectangles just make sense.

If you had any reason rectangles wouldn't be a natural choice it why another shape is better, or a historical example of any traditional writing surface that's not rectangular, we'd love to hear it.