There could be several reasons.
- You are underqualified for the position that they are looking to fill.
- You are overqualified for the position they are looking to fill
- Other applicants have more experience/training
- Other applicants do better during their interviews
I mean there's many reasons why they aren't calling you back, so many in fact that we could literally be here all day listing them all. Times are tough right now and many people are applying and fighting for jobs so businesses are in the position to be able to be extremely picky. People apply for jobs that are less than ideal for them only because they need a job and they are willing to do anything for a paycheck.
For example, I have a friend that is a Kindergarten teacher and because of budget cuts she was laid off. She tried to find a teaching job but couldn't so she started applying at daycare centers even though she's way overqualified to do that. She was hired as a master teacher right away because she has so much teaching training and experience. She said that the daycare center only had the one position to fill and they had over 150 applicants. She got the job pretty much because she was the only applicant that had ever been a real teacher for the school system. There was nothing wrong with the other applicants, she just had more bonuses to offer KWIM?
I think that if you really want to know you should call and ask the hiring manager and ask them yourself. Only they will be able to really answer your question.
"Hi my name is _____ and I applied for the ____ position. I had an interview last week and did not receive a call-back. Have you filled the position? (if they answer yes) I see, well thank you for considering me for the position. I was hoping to get some feedback from you, if you have the time, so that I can somehow make improvements. Can you tell me what was lacking in me that made you choose a different applicant over me?" etc.
The key is being very polite. You never know, they might be impressed that you really care about improving yourself that they may keep you in mind for a later position. The tone of voice and words that you use are extremely important when speaking to a hiring manager. Good luck.