Eve, I would like to offer a different slant on this thread.
You were cut and you feel bad. It has happened to so many, please know you are in the company of some wonderful young women. Will you ever know WHY? I would tend to think not. On the surface I would agree with the possibility of confusion over your status and then, the age difference between you and other freshmen girls. I really want to know WHY you wanted to be in a sorority. Was it for sisterhood? Was it to see what it's all about?
Most (not all) women going through recruitment are DYING to be in a sorority and that emotion along with the giddiness of being a freshman is a very energetic quality. Maybe the actives couldn't read that in you.
Are you in a town with a Junior League or Junior Forum? Many are now on a volunteer basis. This too is like a sisterhood-just no mixers or exchanges with the Rotary or Young Men's Business Club!

Another good source if you aren't 22 is Big Bothers and Big Sisters. BOTH the League andBB/BS will take you far in the business world and because there aren't any specific letters attached, it will set you up socially as well. I wouldn't sit around worrying about the "whys" of this, I would find something else that suits your age and educational status better. Of course as pointed out, seeking alum status in a sorority could work as well, but sometimes it's good to look at the bigger picture. Being exposed to people your age and OLDER will do far more for you at this point in your life. Like Greek life, they are demanding of your time, it takes serious commitment but rewards you every day of your life.