The problem is with how the question is phrased - "one of the biggest mysteries" sounds like clickbait, because in reality Microsoft does periodically explain its decisions, it's just that these explanations don't always reach regular users. It's usually about design or architectural choices that seemed weird until you learned the backstory.
The most common version is about the blue screen of death (BSOD), which stuck around for historical reasons, or about compatibility with ancient software that slows the system down. But here's what people often miss: the "explanation" is usually just an engineering compromise, not some great mystery. Microsoft sacrificed speed for compatibility or convenience, and it was a deliberate strategy, not a bug.
Without the exact source of the news, it's hard to say what exactly was meant. If you remember what it was about specifically - tell me, we can discuss it in more detail. Because Windows really does have plenty of "mysteries," and explaining one of them isn't always the revelation of the year.