Hiring a candidate is like going on a date. It's a mix of background, personality, appearance, communication skills, confidence etc. And like a date each interviewer's opinion of you will be different.
And so asking feedback (a) is pointless since it may force you to change things that don't need changing, (b) clearly demonstrates you weren't the right candidate which may hurt further chances and (c) may be counterproductive since the interviewer may lie or be uniquely identifying faults that others don't find.