Why might an employer decide not to hire you? Many reasons.
It may be that she found someone more qualified or who met certain specifications you didn't possess.
Sometimes it is as simple as that.
Don't beat up yourself too badly if everything seemed to have gone right and you still didn't get the job. It may have. It just went better for someone else.
There are many reasons why job seekers who make it to the interview stage don't end up getting the job. Here are a few of the most common ones:
It is reasonable to seek feedback from the employer about why you did not land the job, particularly if you went far in the interview process. That information can help you hone your job interview skills for next time.
But don't expect too much feedback. Employers are often reluctant to share too much -- or anything at all -- with interviewees who did not make the cut, even if they liked and strongly considered hiring them.