Main reason : To keep compatibility with the existing code, since they have done it with C also.
Also have a look at these C++ Coding standards, which presents some generic reasoning regarding the importance of convention.
These links discusses about the naming conventions of C/C++ Standard Library.
- Naming Convention for C API
- C/C++ Library Naming Convention
- C Library Naming Convention
- Why does Java use CamelCase, while C++ doesn’t ?