When I was rejected in November 2006, I sent a sarcastic (but not rude) email complaining about not being given a reason. However, I doubt anyone was offended. Anyway, I won't be doing that again.
The reason is that there generally are no details to give. Usually when we don't invite a group for interviews, it's not because there was some glaring flaw in the application, but simply because nothing in it jumped out at us.
* even attempting to respond (however briefly) to the deluge of requests would overwhelm us to the point that we could no longer focus on running the core business