Why is the C++ STL so heavily based on templates and not interfaces?

It's called the "C++ Standard Library" (http://en.wikipedia.org/wiki/C%2B%2B_Standard_Library), not STL which is the precursor.

STL and the Standard Library have common elements, but on the whole are different things. People seem to make a habit of confusing them in part because educators insist on referring to STL in their training materials.