maybe for the same unknown reasons that people use the age old "it's not you, it's me" when they're dumping someone?
but yeah I agree with you, it would be helpful for the candidate if they were told exactly why they failed. but trying to think from the company's perspective... I may be dumb but I can't see how it would benefit the company to explain in detail to every applicant who wasn't successful. I mean why bother expending their time and energy. OR, maybe they just can't find a tactful way of telling someone in a decline letter, "honestly, your attitude reeked" or "your dumbass failed the IQ test by a margin of 2000 points." or maybe even "you smelled bad." point is, meeting the required minimums is the least of it, lots of people meet the mins and so there many other reasons why a specific company might not want you.
I think in many cases, people can usually figure out for themselves why they didnt get hired. I mean just think back and see what must've gone wrong. I interviewed with my top choice regional earlier this year and didn't get hired, and even though they never mentioned why, I know exactly what I had done to mess up my chances. (realized it only after the fact though.) It's just a matter of being honest with yourself.
maybe you didnt have that one thing this company wanted, but you could be a perfect match for another.