First, most everybody has a JS engine handy, right in the browser of his choice. JS is reasonably standardized - the subset of it which is common between all browsers is pretty big. Then, JS is rather well known, since all web development has to include that - and that's a sizeable part of all development. Next, the language itself isn't that bad - you may write procedural, object-oriented or functional code relatively easy. Even better, the implementations are quite good lately, being forced to evolve under the pressure of demand.
Can you name a language which would best JavaScript in all these points?