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Atlassian bought Bitbucket after it was already mature. That's why!

Not quite. Bitbucket was acquired in 2011, only supported Mercurial and was missing a lot of features, including the pull request available today.

Perhaps they didn't fire the whole bitbucket team after buying them, so the people were still able to produce good software for a short time.

Bitbucket Server, which some people are referring to here, was build from the ground up, tailored to a self hosting environment.