What to say at an interview when you left on bad terms?

[Nikia]

9. It as if they don't understand that some supervisors and/or companies have serious problems

What if potential employees always asked why they had an opening? If the answer was not that the business was expanding or that the previous person got promoted, than the candidate would turn down the job.
I have actually asked the question about what happened to the previous employee when I was interviewing (although not so much this time). Over 75%, the answer was not the favorable one.
Since it is an employer's market right now, I guess that I understand why if they have several good candidates that they might exclude the one(s) that are not still employed and looking for a better position. One of the jobs where I interviewed, though, has been advertising their position for almost 2 months. As far as I could tell, I was qualified. I am thinking that my answer was why I was not hired. Still, it seems that they rather not fill the position for 2 months than take a chance on me who was not leaving my previous company for ideal reasons.
Good luck to you. I wonder how a company would feel if they were unable to hire anyone for 4 years because they did not have the perfect answer to why a previous employee left.