Do not take this overly personally. Many things could have happened: as you say, they may not have been able to get to know you well enough to decide; they may have had more candidates than places available and had to cut based on a variety of factors; etc. My bit of advice is to keep being involved in campus activities, keep your grades up and build your network of friends & associates. (Depending upon the type of group you're rushing, there may be other things to do or definitely not do.) Keep your chin up -- you gave it a shot and that's important!
