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...
And it would feel just the same if it was named something else.
It's just a name, who gives a damn?