Because its a complex condition. It's not caused by a single factor and is thought to be the result of an interplay between genetic, neurological, motor and possibly environmental influences. Even with the neurological factor, I have a difficult time explaining to clients that it's still debated and the best answer we have is speculative at best. For example, the DIVA (Directions Into Velocities of Articulators) model suggests that people who stutter may have difficulties with the feed-forward control of speech movements, causing them to rely more heavily on feedback mechanisms. This over reliance on feedback can lead to disruptions in speech fluency. However, the DIVA model is just one explanation of how the brain may work in stuttering, and since there's currently no gold standard, I inform clients that research and investigation into developing treatment approaches with higher degrees of efficacy are ongoing. For your own research and investigation, I reference *Stuttering: Foundations and Clinical Applications* by Yairi and Seery, and you can find/download this book online through sites like libgen.