There were far less people who could call themselves designers at the time. It was a slower craft and took more training.
It's like in music. Yeah, there were other bands other than the Beatles and Rolling Stones, but it was harder to produce and distribute music back then. Thus, the industry was more monolithic than it is now. The ones we talk about now are more representative of the state of the industry back then than Lady Gaga (or whatever) is of now.
Further, and here is where it is exactly survivor bias. You have to show that there weren't other people trying to be designers. That the ones you are focusing on are the ones that were successfully doing it, is what makes it survivor bias.