Internal candidates don't always get the job. That's the whole point about advertisng a post, in order to get some kind of competition and to see whether there are better candidates. Even though your experience has been one-sided, many of us can tell you about the delight when we trumped internal candidate and got the job, or the dismay when we thought the job was in the bag which turned out to be anything but. Given the job situation, almost every interview will have one or more internal candidates - those who've been doing a similar job, those well known to the school through supply or placement, and those who are well connected. You just have to ignore all that and do your level best to shine at the interview, and it may just be possible that you have exactly what they are looking for. This could be at your next interview - so unless you go for it, you'll never know.