It’s straightforward: JavaScript is a dynamic language, which allows code (for instance, code implementing an extension to the harness) to be executed and loaded while the harness is running.

This is quite nice — I do think there’s a version of pi’s design choices which could live in a static harness, but fully covering the same capabilities as pi without a dynamic language would be difficult. (You could imagine specifying a programmable UI, etc — various ways to extend the behavior of the system, and you’d like end up with an interpreter in the harness)

At least, you’d like to have a way to hot reload code (Elixir / Erlang could be interesting)

This is my intuition, at least.