The first appearance of System V style init was in SVR3 in the early eighties, before that AT&T Unix took no arguments.
Out of the box, there was not
init 0 nor
init 6 configured; only
init 1 for single user mode and
init 2 for multi user mode.
System administrators were free to use whatever they want with the extra run levels.
With SVR4, run level standardized to these values:
- s : single user
- 0 : halt/poweroff
- 1 : one user mode (!= single user which was more a recovery/admin mode)
- 2 : multi user (machine is a client)
- 3 : multi user; distributed mode (machine is a server)
- 4 : unassigned
- 5 : diags
- 6 : reboot
My best guess is that 6 was chosen because no requirement for an extra unassigned run level was envisioned.
All of this happened before Gnu/Linux was released so anything Linux related is irrelevant.