I'm not an expert, but I have been able to sight read sheet music (to play or to sing) since I was a young kid. From the article and wiki page, I see claims th

I'm not an expert, but I have been able to sight read sheet music (to play or to sing) since I was a young kid. From the article and wiki page, I see claims that the sacred harp system makes things easier, but I can't actually see if or why I might've found it any easier to learn than the standard western notation I did learn. So my guesses are that either it's not actually an improvement, or it's a small enough improvement on a good enough already internationally recognised system that there just hasn't been pressure for this niche form to take over in many places.