I’m not sure 100% why, but I believe it has to do with this invention by Scheibler from 1834. It was the first device to measure pitch (now we use computers) and I guess one of the main reference pitches was 440. I believe he was actually the first to suggest A440. I’ve heard it’s been easiest to synthesize, which doesn’t make much sense to me. Anyway, unofficially, A440 was standardized in 1936 and later officially adopted in 1975.