It appears to be a legacy from TOPS-10. The easy part: octal was more popular in the 60s and 70s in general, but especially at DEC, which produced a number of 18-bit machines; the 3 bits per symbol divides 18 bits evenly, but the 4 bits per symbol of hex doesn't.
CompuServe's beginnings weren't as a bulletin board or ISP, but as a general-purpose timesharing system, where people could dial in and run programs on one of their PDP-10 machines running a Compuserve-modified version of the TOPS-10 operating system. On TOPS-10 a user ID was called a Project / Programmer Number (PPN), and consisted of two 18-bit parts (the project number and the programmer number), each written in octal, and separated by a comma — e.g.
3426,81, and this was how you would log into the system. The practice of giving subscribers project numbers over
70000 was probably a way to ensure separation between subscribers and system users, but I can't find any documentation of that; it's only my guess.
Compuserve continued to use PDP-10s into the 1990s, and as they grew into more of a communications service than a timesharing service, they eventually started using the term "user ID" (UID) instead of PPN, but the format remained the same, and when they connected to internet email, a user's PPN became their email address simply by changing the comma to a dot to get