Because adding additional languages means
I am so very happy that we finally have 1 spec everyone follows. If we start deviating now, browser-wars with missing features and languages will start again.
By the way, it's not like people didn't try to introduce new languages, however even giants, like Google, failed hard, when they proposed stuff like Dart to the world.
No matter what, there will always be people unhappy with the selection of languages, so let's just focus on the important things in one language instead of distributing resources :)