Ask HN: Why is the dot/period character used as file extension separator?

Years ago I worked with Data General minis. The 16-bit ones ran the operating system RDOS (real-time disk operating system), the 32-bit ones AOS/VS (advanced operating system with virtual storage. All used the '.' to mark of stem from extension, but all also had a file type associated in the directory. This meant, among other things, the the assembler would balk at processing something called urgent.asm if the directory showed that it was a generic file. That was baffling the first time you encountered it.

Why not? Maybe it was realized that other candidate non-alphabetic symbols were already reserved for other things, like '/' as a path delimiter on Unices, '\' same purpose on Windows, '#' as a comment delimiter, '%' as the modulo operator, '|' as the command pipe symbol, '^' to signify either power or exclusive-or depending on context ... we're quickly running out of candidate symbols on an ASCII keyboard.