It's all about a persistent connection between the client and server, with the server pushing updates to the client as new data/events come in.

If you don't have a good async model, you're down to a thread/process per connection, which can get very expensive since you will have a thread for every user on your site.

You need something like Erlang's lightweight internal "processes", golang's coroutines, or maybe even Node.js evented I/O async model to handle this well.

all client state is replicated server-side. that means spinning up and down completely isolated threads very quickly. only a stack with immutability to the core and VERY efficient processes (such as OTP threads) can guarantee this while also providing nearly instantaneous spawning from any state (thanks to immutable data structures). Every client that has a page open to your site has a tiny thread running in OTP (it does not use OS-level threads).

No other popular language can pull this off.