There are thousands of people who apply. They can't possibly find the time to reply to all of them. Maybe a few of the borderline ones, but in reality you can't expect them to spend weeks responding to everyone.
And on top of that - most of the borderline projects would probably get an interview if they didn't have so many applications, so even those would be impossible to give a clear reason as to why they weren't picked aside from saying "we thought X other projects were more promising"