As I understand it, in the early days each record company made their own choices about equalization - sometimes in different countries. The choices were driven by performance when these equalizations were being developed. It's much like the various cell phone systems in use today. Eventually the industry agreed on the RIAA curve, but because of legacy records, audiophiles still needed a range of curves for many years during the transition. I have an old mono H.H. Scott receiver with six different curves, selected from the front panel. Also, if there were good stampers in the archives, an old record could be re-published but would not be re-equalized. The rumor mill is full of examples where the actual equalization on a pressing is different from what it says on the sleeve!
Actually, back in the day (early fifties?) everybody used tone controls, so getting the equalization right was not a big issue. Speakers in particular were so far from flat that you needed the controls anyhow.
