Support the runtime, support the syntax and semantics, change the pattern matcher, write extensive tests, add it to the term_to_binary/binary_to_term protocol, add it to distribution, enable reading of maps in various function calls, enable NIFs to inspect maps, define how to copy maps between processes, change the core compiler.

It isn't so simple.

Now if say Nimrod or Dart acquired built-in sets (I don't know maybe it already does) it wouldn't be too remarkable. They are pretty experimental.

Also there is a difference between having a library and a language built-in. All those languages including Erlang, have libraries that provide associative data structures.