So, if anyone could answer this, then I would really appreciate it.
I know that sororities can't share why they chose to reject a girl, but if someone could give me some of the basic reasoning that is used, I would really appreciate it. I really just want an honest, no bs answer to
why I was rejected.
You were not rejected by one, and that one is the ONLY one you need AND the one YOU accepted into your life.
So I just went through Spring Rush. At my school (I don't know if all schools are like this), the only houses that do Spring Rush are those who couldn't fulfill their quotas. There were two houses that needed girls. One of them, the one I worked really hard to get into, went to all of their events, really made an effort to talk to the girls - rejected me. They took most of the girls who came, but they didn't take me. The other one took me, but they literally only met me once before and
I know they did it to improve their numbers. I'm choosing to see this as an opportunity for future possibilities rather than what some people might see it as. But I still have two questions.
Right. Your true and stellar qualities did not shine through, and the house you accepted into your life thought you were the reject that you think you are and bid you
because they consider themselves The Reject House. 
First Question: Why would a house that
I'm pretty sure took the majority of girls that rushed it because they needed numbers not want me? Was I so bad that they felt the need to reject me? I thought I made a good impression and I was really excited to be there. Now I'm really hurting.
As Titchou mentioned, we cannot know. Life is a little knowing, a lot NOT knowing.
Second Question:
what should I do in my current sorority?
What exactly were you going to do in the OTHER sorority? Did you make a list of different things to accomplish based upon which group you accepted?
I don't know what exactly it is that I did that made the other sorority reject me and
I'm honestly really afraid of making this house feel the same way. What should I do?
The home you joined has already accepted you JUST THE WAY YOU ARE, so keep up your GOOD AND POSITIVE ATTITUDE and you'll do fine.
I'm honestly just relieved any house took me and I'm really afraid of doing something that makes this house not want me, like the other one did.
Nope, because YOU are NOT going to allow that to happen.
I will be so thankful for any and all advice.