Also E4X was only ever implemented in Firefox, never really got traction even in Firefox.
But even considering the single implementation problem, it also was just not a good language model, nor was it well specified or defined and it brought with it a pile of weird baggage and complexity.
Then because it was The Future there was no real thought into proper interop with JS (it was essentially a completely independent spec so adopted general syntax but specified in a way that meant JS could not simply adopt that syntax).
But even considering the single implementation problem, it also was just not a good language model, nor was it well specified or defined and it brought with it a pile of weird baggage and complexity.
Then because it was The Future there was no real thought into proper interop with JS (it was essentially a completely independent spec so adopted general syntax but specified in a way that meant JS could not simply adopt that syntax).