Because in some stuff C++ has an edge over C.
Let us summarize some key traits of C++ and C.
+ Much more functionality and flexibility than C.
+ High-level abstracts that makes it faster and easier to create certain type of code.
+ More beginner friendly than C.
- Not as portable as C.
+ More portable than C++.
- Not as much functionality as C++.
- Less beginner friendly than C++.
Feel free to add or disagree with certain cons/pros.