People use LaTeX because it already does a lot of what people want to do. More, it was also one of the first standard ways of packaging extensions for TeX. (I may be wrong on that point...)

Folks are also highly scared off of TeX due to some of its aesthetic nature. Specifically, it looks rather different than most modern languages.

That said, I think most of that fear is unfounded and I regret that I let it keep me from learning this while in college.