Separate issues, I think. Octal was used because that's what most everyone used at the time (a holdover from machines like the IBM 70xx and GE600 series which the early Unix folks were used to). Even in V6 & V7 and later, you see much more octal than hex. Unix perms just happened to be 3 groups of 3 bits. Said another way, we'd likely have used octal to represent them even if perms were 2 bits or 4 bits each.
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Separate issues, I think. Octal was used because that's what most everyone used at the time (a holdover from machines like the IBM 70xx and GE600 series which the early Unix folks were used to). Even in V6 & V7 and later, you see much more octal than hex. Unix perms just happened to be 3 groups of 3 bits. Said another way, we'd likely have used octal to represent them even if perms were 2 bits or 4 bits each.