Although many of us would like to get that feedback, there are reasons not to give specific feedback to job candidates: it prevents you from making any statements that could be held against you (esp. legally); it doesn't take time away from other more valuable work; it discourages people from attempting to hone their interview skills to cover for weak fundamentals; it saves you an awkward moment if the no-go decision was based on personality more than technical chops.

If you really want that kind of feedback, do some mock interviews. What you're asking for just isn't a service that prospective employers have much incentive to offer.