Because it's something wrong with the brain, you could say the same thing about any brain disorder like autism, schizophrenia, psychosy, adhd, add, etc. Also, since it's a tiny minority of humans that have it, then it won't get as much attention and research. And I think the biggest reason is that it isn't that stuttering isn't as bad as other brain disorders as it only affects speech, not actual brain function. If we're lucky, AI can figure something out, but otherwise, I'm doubtful that any researchers would be willing to find the cure for stuttering before something like Autism or Adhd/ADD.