Why do BJJ promotions take so long compared to other martial arts?

In many traditional martial arts, a student can earn a black belt in three to five years by learning choreographed forms and demonstrating techniques against compliant partners. BJJ requires a fundamentally different standard of proficiency. Every technique must work against a training partner who is actively trying to prevent it. This pressure-testing takes time to develop because you must refine techniques through thousands of live repetitions until they become reliable against resistance. The breadth of BJJ also contributes to the timeline. With hundreds of positions, techniques, and transitions to learn, plus the need to develop both offensive and defensive skills, the learning curve is enormous. Additionally, BJJ has a culture of respecting the belt system that discourages premature promotions. A BJJ blue belt should be able to handle most untrained people in a grappling exchange, and a purple belt should be formidable against people from other grappling arts. These standards require time to achieve.