I just realized that my profile was not updated so it is possible that they didn't approve me because of that fact. If that was the case, sorry about that. I have now updated my profile.
Yes, that was exactly the reason. It was not previously clear from your profile what your affiliation with higher education was, if any. Please resubmit your request to join so I can approve it. Thanks for your interest and I look forward to welcoming you to the group.