Probably because Bitbucket is a terrible product. It has the worst interface of the group, and less-than-minimal support for pull requests (comments disappear when unrelated changes are made, aren’t easy to browse, aren’t related to a review; only options are approve and permanently close; code expansion doesn’t work past a certain point, and doesn’t work at all if the pull request comes from a repo you don’t have access to even though you could just merge the pull request and get it).

Also required users making private forks of paid-for organization private repos to pay again to give access to all team members at once last time I used it, but I guess that’s not relevant for open-source migration.