I use HTML/CSS since 3 years and I used often Stack Overflow to solve problems about it. Anyway, I'm a beginner in LaTeX world and I discovered LaTeX site in Stack Exchange. I ask me, Why not use Stack Overflow for LaTeX ? LaTeX is a markup language, HTML&CSS too. Can't you create a site for markup language or ask in Stack Overflow?
This is a summary of the comments, so that we can get an answer. There are a number of reasons why TeX.se exists, and StackOverflow is not the best place for asking TeX related questions.
First and foremost, the Stack Exchange model is based on user choices: new sites can be proposed by anyone on the Area 51 site and if the site gains enough visitors, questions and answers, it gets promoted to being a full Stackexchange site. For TeX.se, this happened almost 5 years ago. The site was proposed, and very quickly there was strong support for it. Ultimately this is the only answer to your question: there's no need to use Stack Overflow since TeX.se exists and is strongly supported.
TeX is not really a markup language, but a fully fledged programming language, albeit one designed with typesetting in mind.
StackOverflow is aimed mainly at programmers, but most TeX users are not TeX programmers, since the goal of TeX is to produce documents, not to program things. The questions that are addressed here are therefore not mainly about programming, but about how to achieve particular document output. Of course, since TeX is a programming language, we also have programming questions, but these are in some sense the minority.