So many comments in the vein of "who has time to waste on candidates we aren't going to make an offer to?" - You are damaging your companies brand thinking this way. You want people who you reject to say to themselves "I can't wait to study up and interview here again, they said no, but I still want to work here!"
I mostly disagree. I really can't think of a case where I would want to interview the same person twice. That said, I'm happy to provide feedback if they ask, and when it comes to interviewing, I personally appreciate feedback ONLY when I ask.
Regardless, brand matters as well. You want candidates to have good experiences even if they are rejected as they will talk to peers about company x