Because it imports everything from the std namespace and can lead to name clashes. It also makes it harder to see where symbols come from. If you see `std::vector` you know exactly what it is. If you just see `vector` in a large code base it could be anything. Could be a hand-rolled version of std::vector from prehistoric times or people not trusting the STL for no reason. Could also be a mathematical vector or something for vector graphics or whatever else. That is why people generally always fully specify where things come from, it's not limited to just the STL.

`using namespace X;` is somewhat fine if you limit it to a scope by using it in a source file and ideally only the function body where you use it, in header files it is a really bad idea because then the namespace gets exposed to everything including that header. That can lead to the compiler calling the wrong function and you might not even notice it.