I'm not saying it's exactly a mystery. Java had the misfortune of being ubiquitous when XP and design patterns became all the rage. Every intern using Java had to learn "patterns", and schools where teaching them too. Heavyweight frameworks like Spring (and before that, EJBs( were all the rage too. It took a lot to extricate modern Java from that situation.
Is the reason a mystery? Java is the language that got a multibillion dollar marketing pitch at its inception. It was designed to be in "the enterprise" from day 1.