Support for other browsers are coming. I am neither working on this nor a Google engineer, but I saw a tweet from relavent developers working on this.
In 2017, with the combination of pretty close parity between VM's technology support (even Chakra) and the range of powerful preprocessors, build tools and shims, if you're not supporting multiple major user agents you're an objectively bad frontend developer. This isn't about the cost benefit of committing time to smaller market shares because the cost is negligible. The only explanation can be incompetence.
... or that you're the maker of said browser and stand to gain from actively deprioritising your competitors. I don't know that this is policy at Google, but it's either one or the other.
Maybe "use Chrome" means "not tested on Firefox and Edge?"