It rhymes better than “steel is sexy”
I have never drooled over a CF or Al road or hardtail MTB frame.
I don’t know about road bikes, but in the mountain bike world, “steel is real” has been a saying since at least the late 90s. Maybe longer. I assume it became a thing not long after aluminum bikes frames dominated the market. It (steel) has steadily become more popular I would say since the mid 00s. When i was looking for a steel hardtail frame in 2006 there were plenty of options out there.