The pressure to evolve JS in a more fair-play standards setting rose and fell as browser competition rose and fell, because browser vendors compete for developers as lead users and promoters. Before competition came back, a leading or upstart browser could and did innovate ahead of the last JS standard. IE did this with DHTML mostly outside the core language, which MS helped standardize at the same time. I did it in Mozilla's engine in the late '90s, implementing things that made it into ES3, ES5, and ES6 (Array extras, getters and setters, more).
But the evolutionary regime everyone operated in didn't "decide" anything. There was and is no "Silicon Valley" entity calling such shots.