std::unordered_map was only added in C++11, which explains why it's not used in legacy code. Habit, inertia and a desire to keep codebases internally consistent explains why it's less common now.
The real question, is why C++ designers chose to prioritize sorted maps (RBTrees) over unsorted maps (Hash Maps).
I don't think it's a matter of speed though. I think it's because RBTrees are safer to use. Things can go really wrong with a hashmap if you have a poor hashing function (worst case search is O(n) vs. O(log(n)) for a RBTree)
[0] : https://www.reddit.com/r/programming/comments/5pwgtn/hash_ma...
[1] : https://stackoverflow.com/questions/42588264/why-is-stdunord...
Honestly, sorted vectors can often be a good replacement, depending on the workload, if you must have ordered data (this is what boost flat_map uses). All 3 (map, unordered_map, sorted vector) can have their uses, depending on what you are doing with the data.
If you need a better performing hash map, there's always Abseil as well.