You just have to look at it from an administrative point-of-view. They probably get lots of applications, with plenty of candidates that qualify for the position. Who would you rather pick? Someone with a clean sheet as far as problems go, or someone who has a long list of problems but promises that they've "changed." Now I'm not saying that you can't change... it's perfectly likely. I'm just saying that it's a much safer choice to go with the many other applicants who you know haven't done anything wrong instead of taking a risk of someones "promise" that can easily be broken when they feel like it... except now have the power to do a lot more damage.