People use github over bitbucket if they like the features that github provides, the diffs, issues etc. Not sure which features are unique to github alone, but each provider (including gitlab) has their own flavor. Many are just used to github flavor.
I use bitbucket at work, and it has many of the same features as github. The UI is a bit clunkier, and it does lack some of the flashy features such as automatically squashing commits when merging a pull request. However it does work fine, and it is well integrated with atlassian's other offerings which we also use. Mainly JIRA and hipchat, though bitbucket also has a per-repository issue tracker.
Could you elaborate? Is it used for every pull request? That would be annoying, but sometimes developer leaves a trail of "work in progress" commits, so I'll wind up squashing those anyway.
