The Big Five Personality Test is more boring because essentially, it’s just naked observation: For example, there is a trait in that system called Openness. And the way that is determined is by observation. So, for example, if Thomas Jefferson liked lots of exotic and foreign foods, then the raters notch up his Openness trait. But if he liked to stay at his plantation (rather than see new sights), then they deduct a few points and so on.So basically, your personality in that system is nothing but the sum of all your actions through life.
In MBTI we say instead that two very different people can do the same thing outwardly, but they can do it for very different reasons inwardly. For example, Johnny Depp and Robin Williams may both “act the fool” in a highly formal interview setup like ‘Inside the Actor’s Studio.’ If we used the Big Five system, we would say that the two are essentially the same. But if we used Jungian typology, we might say that one person acts this way because he really enjoys an informal good time (Williams), while the other person is emotionally uncomfortable with the setup and uses pranks as a way to get away from it (Depp).
So there is more freedom in this system and a bigger element of interpretation involved. This makes the system less scientific, but more interesting to most people.
So that’s why MBTI is more popular than Big Five, even though Big Five is more rigorous.