Because in practice, isn't this a bit like "Kleenex" - where everyone knows you mean "tissue" (EMCAScript).
JavaScript is simply the better term, and marketing is everything. Reminds me of Java's POJOs, which was a very simple pattern that no one used, until someone gave them a fancy name.
ECMAScript is a horrible technical name. Might as well call it ACMEScript considering how willie e. coyote it feels to develop with it...