Probably because allocating a thread to every request is less efficient then a bunch of event loops with async. Ideally you do both, but a lot of languages cho

Probably because allocating a thread to every request is less efficient then a bunch of event loops with async. Ideally you do both, but a lot of languages choose to make concurrency something done at the process level and are event driven asynchronous web servers which are deployed as multiple processes, which adds a lot of overhead compared to threads. I think Golang has a nice balance with goroutines, as I understand things.