All of this was hashed out during the "Harmony"[1] days. Versioned-JS was of course one possible future. Maybe even still is. But the prevailing decision coming out around that time and leading to ES5 and ES2015: We'll add "use strict" as a single-point-in-time breaking opt-in upgrade to fix a lot of the common problems, but let's otherwise stick to "One JavaScript"[2].
You may find [2] and [3] especially enlightening to understanding this thinking, and any other discussions from ES Discuss on the topic if you fell like digging into history.