YC can't and shouldn't provide reasons why. They've already said as much. In their case, it's different from Fred Wilson. YC has to say no just twice a year. For Fred Wilson and any other VC, it's an ongoing thing. There's a difference.
Most VCs (and I've never met Wilson, but I'm guessing this applies to him) meet people in person, because they've gotten introduced or referred by someone in their network.
So in that context, it's tougher to reject people without providing more details.
As I've explained over and over, the reason we don't provide more details is that there often are no details to provide. Often the sum total of our reaction to an application is "we don't understand this."
The main difference between us and Fred Wilson in this respect is that he might evaluate 1 or 2 carefully screened deals a day, while we evaluate several hundred.
Although I understand many of the reasons are probably due to simply not being compelled by the founders, which is much more difficult to tell someone than simply that "there are too many competitors".
The truth can hurt, but sometimes that is what someone really needs to hear.
Is it the lack of communication skills conveying the idea? Or is it something more fundamental along the lines of, "we don't understand this 'foo'"?,
Where "foo" is 'idea is too derivative', 'you have no users in mind'... etc?