I don't know how it started, but "concealed weapons" were evil when demonizing them was a useful way to justify making them illegal. I still recall, in the mid-90s, that people still argued if you were going to carry a gun for self-defense, it had to be visible, so everyone knew you had it. Pretty funny that that's now considered rude, or threatening. Now, people seem to think there's an obvious moral duty to conceal your gun the way a man has to conceal, ya know, his other equipment.
It's dumb that we have to use the words "conceal" and "concealed" so much. There's a million articles and videos and podcasts talking about "concealed carry," when the conversation could just as easily apply to weapons that aren't "concealed." And why "concealed" or "open"? I have a flashlight in my pocket right now, but no one would say I'm "concealed-carrying" a flashlight. I have a pen clipped into the front of my shirt, but no one says I'm "open-carrying" an ink pen.