It's because what the "internal monologue" is to each of us probably depends on how each our minds adapted differently to facilitate tasks (for example, I always had to mutter "negative b plus or minus root b squared minus four a c over two a" in my mind when using the formula on my math assignments). The internal monologue is also helpful for complex reasoning, but for those who aren't prone to complex reasoning it might not be as necessary, and so it would be less likely to develop. It probably correlates to IQ, but I realize I sound like a twat for saying so.