I think Scylla is a bit of an outlier in terms of software quality :) [I mean that as a compliment]

We do parallelism across CPUs and nodes. We run single-threaded to get the most out of a CPU in a shared-nothing architecture. Many single-threaded apps aren't written to really take advantage of all a CPU has to offer. But there are also prices to pay to run multi-threaded; context switches, etc.

Not having (thread-level) parallelism encourages designs with small, well defined interfaces where data crosses threads. In a typical NodeJS server setup with one load balancer and a number of node processes that don't know about each other you will experience fewer concurrency bugs and care less about NUMA than in the equivalent ASP.NET app (requests scheduled onto a thread pool, code can share resources at will).

Of course sometimes "small interface" isn't really viable from a performance standpoint and you want a well-engineered multicore application with lots of shared data, and you just can't do that with JS or Python (or at least it's very hard). That's a good reason to choose a different language.