1) He had a HUGE amount of personal charisma. Some lecturers are watched because they know a lot or are famous, despite a lack of public speaking skills. Feynman could have gone into acting or politics the guy is genuinely entertaining and a VERY skilled presenter. Feynman's on camera personality is the professor from Gilligan's island but funnier and friendlier.
2) He got his Nobel price in peak boomer years 1965 and then didn't die until the end of the 80s. For boomers he is "their" generation's physicist just like the WWII gen had Einstein as "their" physicist. Who is "the" popular science fad physicist for the X-ers and younger? Hawking, maybe Susskind, possibly even Sabine, I guess?
3) IMHO he was an autodidact who wrote for fellow autodidacts. That is my learning style. His style REALLY STRONGLY resonates with me and my learning style. If you're capable of self-teaching you get a feel for who's your type of author and who is not. Feynman definitely writes books for people like me. His books and notes are all old, of course, which is sad. As for "moderns" who emit similar intense autodidact vibes, I'd suggest Schroeder and his famous "Introduction to Thermal Physics" from the turn of the century. I subjectively like that book. I don't care if there's a better way to learn bachelors thermodynamics by taking a course in a classroom or watching video lecture, I just like the book's style. Not the superficial style like typography but the organization and connectivity of the topics is very autodidactical, just like Feynman's books. To some extent, he's post-education in that once you are done officially learning, the rest of your life you're an autodidact, like it or not, and Feynman's style leans into that. I still remember as a kid in high school, where I took two years of public high school physics, paging thru a copy of Feynman's lectures in the library and it was so clear and so fascinating compared to my experience in "official classes with new textbooks".