Before 1970, gay men (and to a lesser extent women) really couldn’t be out, and even until the 1990s it was extremely risky in most of the country. You could be arrested for it, gay hangout spots were often raided, being outed typically meant losing your job and maybe family, and you might get beaten/killed if you mentioned it to the wrong person. Because of this, there was a whole semi-secret culture involving gay bars, coded messages, and subtle signals for gay people to meet. In a leather bar, for example, showing up in the correct ‘uniform’ (something like boots, plain jeans, white t-shirt, leather vest or jacket) told people that you were safe to approach, while showing up in normal clothes warned people that they needed to be careful of you, that you might be a cop or might out the whole place. The BDSM leather subculture was an outgrowth of this environment of secrecy, combined with an attitude along the lines of ‘well, if I’m already rejecting normal sexuality, I may as well embrace the other deviant stuff I like’.
Straight people didn’t have a need for all of the secrecy, so didn’t have the base of special locations and signaling to draw on. It was a lot easier for a straight person to pass as completely normal and only get up to the ‘freaky stuff’ later on in a relationship, or as part of hooking up. AFAIK most of the non-gay kink subculture really only took off in the 1980s and 1990s, before that there wasn’t really any culture of meeting beyond individuals and small groups.