Why are leading zeroes used to represent octal numbers?

Worth noting that in Python 3.0, they decided that octal literals must be prefixed with '0o' and the old '0' prefix became a SyntaxError, for the exact reasons you mention in your question

https://www.python.org/dev/peps/pep-3127/#removal-of-old-octal-syntax

All modern languages import this convention from C, which imported it from B, which imported it from BCPL.

Except BCPL used #1234 for octal and #x1234 for hexadecimal. B has departed from this convention because # was an unary operator in B (integer to floating point conversion), so #1234 could not be used, and # as a base indicator was replaced with 0.

The designers of B tried to make the syntax very compact. I guess this is the reason they did not use a two-character prefix.