They've become a parody of themselves.
What makes a great rapper great is often a combination of the intentional and the unintentional. When an artist is clicking, there's an essence to their output that is greater than the sum of its parts—overarching inspiration that goes beyond any definable skillset and can't be forced or constructed. Think about conversation with a highly self-conscious person: crippling, cringe-worthy. Once rappers become too aware of how they're audience perceives them—what they think people like about them—they often start to write themselves into corners. What had been a fully-fleshed-out persona becomes caricature, and loses all the subconscious ripples and tics that makes art so compelling.