I think it's the relative obscurity of the 396 engine, in general, that has held back the popularity of the Camaro BB cars. 350 is a legend, obviously. 302/Z is a legend, obviously. Then, the really cool GM engine of the day in the BB variety was the 427 in the vette ... and through dealer or COPO modification in the Camaro (and I don't know enough about Nova/Chevelle/Chevy II to know if/where that engine played over there). In 1970, the 454 came out and that was that. 302 - 350 - 427 -454 ... and nobody really remembers the 396 (and, by "nobody," I mean the casual fan, not the Camaro or GM enthusiast). So, I think it's taken time for the market to recognize the rarity and the value of the 396 1st Gen Camaros.
I liken it a bit to the Mopar cars. They weren't worth much more than their weight as scrap metal up through the early 200s. Then, all of a sudden, people remembered that those engines were absolute monsters. I still think the styling isn't in the same zip code as Mustang and Camaro (and Corvette, Nova, Chevelle), but the market eventually remembered the rarity and power of those monster Hemi engines ... and their values went through the roof almost immediately.
In some ways, I think that's a similar story to the L78, L34, L35 BB 1st Gens. The market (casual people) just haven't paid enough attention, but that has changed and is changing.