Have you tried asking them whey they didn't select you? If you express genuine disappointment and ask what it was about you that didn't fit what they were looking for, there's a reasonable chance they'll give you feedback on the interview.
In general they don't tell me when I do ask though. I understand that; they don't want to risk me suing them if they divulge anything that could be even construed as illegal, I probably wouldn't give feedback either honestly.