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).
This is a really disturbing and sad note of kickboxing. It declining so much faster than other MAs. Kickboxing has no popularity and growth like other martial arts. If you compare this with boxing Muay Thai or BJJ. We seriously have to discuss this.