You'll probably never know exactly why you didn't get the job. If you're like most job seekers, it will remain an unsolved mystery. Most employers do not provide useful feedback for fear of legal ramifications or other hostile actions on the part of unhired applicants.
That said, there are typical reasons that may explain why you did not get the job:
Your resume didn't make a clear case for your qualifications. You did not articulate a connection between your skills and accomplishments and the employer's needs. It's likely a computer scanning system, known as an applicant tracking system, reviewed your resume for keywords and phrases necessary to match you to the employer's needs. Even if it is very clear to you that you're well suited for the job, it's your responsibility to make sure you demonstrate your skills and accomplishments in your resume. Do not rely on a cover letter or any other documentation you may send with your application to describe key reasons why you are well suited to the job. It's possible your target employer will not even see that information until you pass the resume screen.There were grammatical or spelling errors on your application materials. This complaint often comes up when employers and recruiters gripe about job seekers. Especially if you are applying for a job that involves writing letters or correspondence, such as an administrative assistant, one typo or error can make the difference between landing an interview or not. It's difficult to proofread something you wrote yourself, so consider asking an eagle-eyed friend to review your cover letter and resume before you send them in. Alternatively, try to read your resume backwards. Of course, always spell check your work, but be aware that spell check doesn't pick up every error. You were unqualified or overqualified. This is another common recruiter complaint. Many job seekers apply for every job that seems reasonable to them, but employers may consider the candidate ill-suited to the job. As a rule, do not expect interviews as a result of applying for positions you are overqualified or not quite qualified to do. Employers filled the position internally. Sometimes, employers advertise jobs they intend to fill internally. When this happens, you never had a chance at an interview, anyway. It's frustrating for job seekers, but an unfortunate fact of job search.