More than likely because you dont have the template tag for displaying it in your sp theme..  As shipped by us, the template files show the badges for the forum ranks and not the membership badge…  to show both would be overkill and look poor so we had to pick one…  just happens you want the other one…

So you need to edit your sp theme… we recommend making your own theme before making any changes so you dont lose the changes… see:  http://codex.simple-press.com/codex/themes/theme-basics/creating-a-theme/

Since the badges appear on a topic view,  you want to customize the spTopicView.php template file…  find:

                            sp_PostIndexUserRank(‘tagClass=spPostUserRank spCenter’);
                            sp_PostIndexUserSpecialRank(‘tagClass=spPostUserSpecialRank spCenter’);

in there… and you can either replace those or add the membership badges to it with:

                            sp_PostIndexUserMemberships(‘tagClass=spPostUserMemberships spCenter’);