I was recently in London for work and went to the Marylebone farmers’ market mostly to sample apples. I was struck by how little overlap there was with the varieties I know from the US. Cox was the only one I recognized.

Why are the varieties so different? Is it mainly climate, or did apple growing simply diverge on either side of the Atlantic?

Short answer: Both, but the history matters more. Early American orchards were often grown from European apple seeds, and each seed produced a new variety. Growers kept and grafted the seedlings they liked, while British growers made their own selections. Climate shaped which apples did well, and local market stalls make that long divergence especially visible.