Maybe because the dev.to domain was available, and the others weren't ... apart from that I think it's a good choice - "io" is mostly associated with tech companies which want to sound cool and "hipster", "com" is too commercial, "org" sounds bureaucratic, etc ...
But my main hypothesis is just because the domain was available, and didn't sound weird.
Cost may have been a factor too, I guess "dev.com" even if it was for sale might have been pricey.
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