It may be myopic, but the only major reason I can think of is visibility if you plan on publicizing the repo at some point. But you can achieve this by using GitHub as a mirror for your main repo on another platform (at least for GitLab, I’m not familiar with BitBucket).
Edit: Personally, I use GitLab (both .com and I've set up/maintain several sefl-hosted solutions). I originally tried out GitLab due to lack of private repos on GitHub, and I don't see a reason I should switch back at this point. The open development and attitude towards the product and community has been part of that.