Why do we only use C in data structures and not C++?

Data Structures are particular way of organizing data in a computer so that it can be used efficiently. (Wikipedia Definition)

So you can not state that it must be done only in one programming language, like C or C++ or Java etc.

It is very important for you to know that, there are lots of different usages of Data Structures which can be in connection with the programming language (as a usage ease), but this isn’t mean that you can not do any similar thing in Java that you’ve already done in C++ when you use Data Structures. You can do all the same things in any language that supports data structures.

There is a reason why they use C language when they give you examples in school or in blogs. C has very special way to give you the logic behind the Data Structures, just because it is NOT an Object Oriented Programming. If you look into it a bit, you are going to see that there is a huge difference between Data Structures in C and C++/Java etc. OOP based languages are automating the functions of C language (using it’s pointers, predefines etc.) in Queues, Linked Lists, Trees etc.