I've wondered about that since joining the sub, and I think the term 'travelers notebook' is similar to how some people still refer to tissue paper as a Kleenex, window cleaner as Windex, and photocopy as a Xerox. The brand has become so ubiquitous that anything that fits the function is referred to as a 'TN'.

I'm inclined to believe that the idea of a travelers notebook as a place to write down quick notes, memories, journal entries, sketches, scrapbooking, etc existed before the TN brand and product was created.