There's a pretty good/brief Wikipedia article about it: https://en.wikipedia.org/wiki/Concert_pitch https://en.wikipedia.org/wiki/Concert_pitch (see specificall

There's a pretty good/brief Wikipedia article about it: https://en.wikipedia.org/wiki/Concert_pitch https://en.wikipedia.org/wiki/Concert_pitch (see specifically "Pitch Inflation" In short, it was competition for the performance to seem more "bright" and exciting. It's arbitrary, but for performers to reach higher pitches was viewed as more challenging (and it is, for singers). The wiki article is cited, so you can read up in as much depth as you like. But, it mostly boils down to, "That's just what people liked during important periods of musical development. And, then we stopped because technology advanced to the point where we could very accurately determine pitch and for the past 50+ years the majority of recorded and performed music has been A=440, or very close to it."