A lot of groups who don't get invited to interviews would like to know why. This is a reasonable thing to want. If there's something wrong with your project, you want to know what.
So why don't we tell people why we didn't invite them to interview? Because, paradoxical as it sounds, there often is no reason. The median application is usually pretty good. The reason it gets rejected is not that it seems particularly bad, but that there are a sufficient number of others that seem particularly good.
Probably the reason people expect feedback about why they were rejected is that they implicitly think of this as like a grade in a class. But a test where only a fixed number of applicants can pass regardless of the average quality is not a grade in that sense.
There are physical limits on the number of groups we can meet in person. We can only interview a fixed number of groups, no matter how many good ones apply. From that cutoff down to about the halfway point, the applications are pretty good. The reason they didn't get invited was not that there was anything specifically wrong with them. They were just pushed down by other applicants who were particularly stellar.
So the reason we can't respond to emails about why groups were rejected is that a lot of the time there's literally no answer. We could make one up, but we'd be lying in many cases, and the better the group, the more likely we'd be lying. The main reason the top third or so of the applicants don't get invited to interviews is literally not to be found anywhere in their application, but instead is distributed across the top few applications that pushed them down below the cutoff.