It's a movie about physics, with characters like Niels Bohr featured prominently, which just so happened to be made for general audiences and it was a hit, by a director whose other historical film was about Dunkirk.

Before these movies were made, the subjects were pretty much obscure to the mainstream. Films like these are regarded as risky for large studios, and it's widely acknowledged that Christopher Nolan is on the very short list of directors with the blessing to do absolutely whatever they want at large studio scale and budget and that is not part of a franchise. And by "very short list" I mean people like Stanley Kubrick.

The "mainstream" label on Oppenheimer is incidental, after the fact.