Feynman had the vanishingly rare combination of being at world-shaking (literally) historical events like the Manhattan Project and the Trinity test that ordinary people can relate to¹, actual scientific contribution enough to merit a Nobel Prize in Physics, and being an engaging storyteller and educator. I don't think it's all that surprising that his recognizability very high.
¹ Ordinary people can relate to a giant industrial project and a huge boom. They cannot relate to some person sitting in a room writing arcane symbols and muttering to themselves until one day they yell "Heureka! Ich hab es gefunden!" or whatever the proper German is and rush off to publish a paper.
¹ Ordinary people can relate to a giant industrial project and a huge boom. They cannot relate to some person sitting in a room writing arcane symbols and muttering to themselves until one day they yell "Heureka! Ich hab es gefunden!" or whatever the proper German is and rush off to publish a paper.