How comes over 25% of retractions fail to explain the reason of the retraction?
One reason is that it can be a face saving technique.  Sometimes there are suspicions of fraud or other misconduct that are difficult to prove, or the authors strenuously disagree with each other about who had responsibility for something or exactly what happened.  Instead of fighting about the explanation, the editors and authors may agree to retract the paper while giving only a vague reason that everyone can agree to.  This is not ideal as far as scientific clarity goes, but sometimes finding the truth is genuinely difficult, and even when it's possible it may not be worth the effort.

As a general heuristic, if a paper is retracted with no clear reason, then the reason is probably not good (or at best it's complicated or debatable).  After all, if there were an explanation that sounded good and was clearly true, then why wouldn't they include it?