harkening back to the original
PDP11PDP-8 (and possibly earlier) days
Well, C was originally developed for the PDP11, wasn't it? Octal itself is certainly much older, but I hypothesize that its inclusion in C was directly because C was aimed at PDP11 "systems programming", and the PDP11 was defined/documented/etc in octal.
(The 8080 also had a fundamentally octal instruction set, and it always pissed me off that hex was so widely using for programming it.)