> Multiple languages coexist. Within Javascript, there are multiple module systems, general utility/polyfill libraries, test runners... for better or worse, we continue to have many alternatives for almost anything. So why not also different implementations of a language interpreter?

As it was already said they run the same package manager and claim compatibility. This means you either can't take advantage of any new API in io.js ever to allow your module to work in both or you take the road where you're transpiling to support one and the other. It's not sustainable and it's prone to issues.

Because they run the same package manager and roughly do the same thing for the same purpose? It's not a JRuby vs Rubinius debate, imo; it's more like YARV vs MRI, you just don't need both. If they were more different I'd agree.