[actsasbuffoon]

If you've ever used EventMachine in Ruby, or Twisted in Python, you've probably encountered the reason.

Blocking has simple semantics, but it's hard to scale. Everything-is-async is a little more complicated, but scales nicely. The problem is when you combine the two, you only get the worst of both approaches.

If most things are async, and you've only got one thread, and you block that thread, then your whole app is blocked. Now you've got the scaling problems that come with blocking, and the complexity that comes with async.

[Lerc]

Well preemption and blocking should go hand in hand. The distinction of whether or not that's multi-threading is more of a semantic argument. I'd consider them the Amino acids that the multi-threading protein is made of.

The entirely async model means that if something accidentally endless loops, it kills everything. The only reason why we get the "This script is taking a long time" is because some preemption is provided as some magic beyond the scope of the JavaScript itself.