1. The prefix is useful to know.
 2. The full list contains distractions and makes the page feel less authoritative.
 3. New/future projects aren't likely to go into JIRA.

The `HR` project in issues.civicrm.org was used for CiviHR 1.0-1.4.  When
the project was transferred to Compucorp, they moved issue-tracking to a
private system.  The JIRA `HR` project only exists for historical purposes.

https://github.com/civicrm/civicrm-core/blob/master/.github/CONTRIBUTING.md