Looking at the two data structures and algorithms to handle them, a hashmap is not really any more complicated than a binary search tree and possibly less complicated. And the hashmap has the advantage of constant time access of a key. So why did we get a std::map very early on in the history of the standard library but for std::unordered_map which is a hashmap we had to wait for C++11 ?
C++ Binary Search Tree vs Hashmap – Historical Reasons and Simplicity
algorithmscdata structures
Best Answer
Implementing a hashmap is much more complicated, not in actual implementation but in the possible variants you have to pick from:
All of those affect performance in a way where it's not easy to say which one is better in general.
Compared to that the red-black tree is much simpler to create a prototype for that people won't bikeshed endlessly about.