The problem is that there is really no such thing as a pull request.
A pull request is a hack, an illusion created by these clever remote hosts like GitHub and Bitbucket. It's not a Git feature, and it isn't part of the repo in any standard sense.
So when you copy the repo (e.g. by cloning), there is no "pull request" in it. You can see the branches that were created in order to request that they be merged, but you cannot see the "pull request" per se, as it is not a thing as far as Git is aware. The pull request and the code review are part of Bitbucket, not part of Git or the repo.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…