Great candidates often choke under the pressure of interviews. Among well-qualified candidates, at least a third (and maybe half) of all interviews are ruined by self-inflicted wounds--preventable mistakes that a job seeker could have avoided with better preparation. An interview is a "make or break" moment in your career, and far too many people handle it badly.
So, assuming you are being interviewed for a job that fits your qualifications, why won't you get it?
You walked in lightly informed, but not well informed. You spent about an hour preparing for the interview, but you should have spent three. So you could not relate your experience to their needs, because you did not yet understand their needs. Insufficient preparation is the biggest interview killer, and it causes the next two major mistakes.
You did not directly and forthrightly answer their questions. You did not think about what kinds of questions you would be asked, and you did not prepare clean, concise, authentic answers to their predictable questions. So you ended up talking too much and saying too little. This is the second biggest killer of interviews. (And here's what you can do about it).
Finally, when given a chance, you asked the wrong questions. You did not demonstrate curiosity about the organization, their challenges, or how you could help. Your questions were either simplistic, dull, or all about you. Asking, "Do you have flex-time?" will not endear you to the hiring manager. You can always ask those kinds of questions after you get the job offer. Before you get the job offer, you want to be asking questions about how you can make a significant impact on the job ... giving them a reason to hire you. Curiosity is the one attribute that smart employers look for, but job seekers rarely demonstrate.