the first version of bluesky was an internal effort to make twitter decentralized. atproto comes from twitter/ig type social media where almost all content is public so not supporting private data for v1 was an obvious choice.

they only decided to expand into other domains after the elon takeover made people move away from twitter and bluesky got popular way faster than expected.

> obvious choice ... decided to expand into other domains after

I'm not sure what the obvious answer is. I imagine the user insights at Twitter would be very informative, but was never on the inside to know. It's also worth noting that many members of the team joined on after ATProto was broken out from Twitter. Only they can provide an accurate recount of the information they had at hand and the decision making process around it.