I don't know why. VMS also used it to indicate the revision number of a file, as I recall--demo.c.1, demo.c.2 ...--but it has been a while.

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.