It is an interface compatibility issue. Basically, the design is done considering the interface of std::map.
In other words, for std::unordered_map it does not differ a hint is provided or not.
Additional Information from the comments here:
The interface compatibility is very important because being able to quickly/easily switch between map and unordered_map provides the valuable flexibility of painlessly transition since performance is often the deciding factor in choosing one over the other.