I've added my GitHub account to the remote repositories in Sourcetree. Much to my surprise, I cannot see my private repositories.
After some research, I found out, that the API call to https://api.github.com/users/<username>/repos only lists my public repositories - and this is what Sourcetree seems to call.
A call of https://api.github.com/user/repos?access_token=512295a0afb73bdd1c076a00c69f8abcd12345 (fill in a valid Access Token) instead shows all public/private repos.
Can you please add the option to specify an access token, so that private repositories are listed as well?