It's just not good. Like asking why handball isn't a global attraction. The participants just aren't taking it as serious as major competitions like boxing, basketball or even MMA. Kickboxing is adjacent to karate, "training," sure, but not exactly in a milieu that breeds real talent. I know this is hard to digest. But if you disagree then truly consider the plateau that sports like kickboxing and MMA constantly hit. There's a handful that rise above the crop but it's always short-lived or gimmicky (Tenshin, billed as teenage hearthrob pop-star).