It's just my expereince really. Qualifications get you in the door for an interview but there is a huge difference between a piece of paper that suggest you can do the job and actually being able to do it. Solid evidence of ability is a good way of showing you can actually DO the job you are being hired for.
Obviously there are loads of reasons people do/don't get hired but there I have always found there to be a certain amount of suspicion over what it is your CV. e.g "Team player" O RLY? etc..
Out of interest why did you hire them? Simply had the right qualifications? Right personality?