Why there are not all confluence plugins available at https://packages.atlassian.com/maven-public/com/atlassian/confluence/?

I need the source of

https://jcenter.bintray.com/com/atlassian/confluence/plugins/confluence-jira-plugin-parent/
but get forbidden when I try to access it.

Sadly in the public reachable repository there is no equivalent entry under
https://packages.atlassian.com/maven-public/com/atlassian/confluence/plugins/

anybody knows how to get access?