Because browser bugs, platform features, and language features have always been a mish-mash and a set of moving targets. See the ES6 language compatibility tab

Because browser bugs, platform features, and language features have always been a mish-mash and a set of moving targets. See the ES6 language compatibility tables for a visualization ( http://kangax.github.io/compat-table/es6/ http://kangax.github.io/compat-table/es6/ ). So, the choices are either only use features once you _know_ that a large enough percentage of your target audience supports it, or transpile newer syntax into older, more widely compatible syntax.