Boost is used so extensively because:
- It is open-source and peer-reviewed.
- It provides a wide range of platform agnostic functionality that STL missed.
- It is a complement to STL rather than a replacement.
- Many of Boost developers are on the C++ standard committee. In fact, many parts of Boost is considered to be included in the next C++ standard library.
- It is documented nicely.
- Its license allows inclusion in open-source and closed-source projects.
- Its features are not usually dependent on each other so you can link only the parts you require. [Luc Hermitte‘s comment]