If all of HN has only a single core then you're running it on less server resources than I could buy on ebay with $180 and a visa card?
For another data point, I've got a single box processing every trade and order coming off the major cryptocurrency exchanges, roughly 3000 messages/second. And the webserver, DB persistence, and a bunch of price analytics also run on it. And it only hits about 30% CPU usage. (Ironically, the browser's CPU often does worse, because I'm using a third-party charting library that's graphing about 7000 points every second and isn't terribly well optimized for that.)
Software gets slow because it has a lot of wasteful layers in between. Cut the layers out and you can do pretty incredible things on small amounts of hardware.
You see, it is possible. Take a step back and look again to see the evidence: The site is called Hacker News, the first goal of this site was to prove that something useful can be created in an entirely custom programming language, and the site handles a huge amount of traffic, so it is a challenging task to let this run on a single core.
So the answer from a hacker's mind to why they let it run on a single core might simply be: Because they can.
On the other hand, YCombinator is a successful company, so buying a larger server would certainly be in their latitude. But that would be less intellectually appealing, and part of their success come from the fact that they decide as hackers, and don't always take the easiest path.
RAM is so cheap now for small sized things that you can afford to trivially have an entire db cached at all times, with only very rare disk I/O.
As an example we have a request tracker ticket database for a fairly large sized isp which is a grand total of under 40GB and lives in RAM. It's dozens of thousands of tickets with attachments and full body text search enabled. For those not familiar with RT4 it's a convoluted mess of Perl binary scripts.
I could probably run my primary authoritative master DNS on bind9 on Debian-stablr on a 15 year old Pentium 4 with 256MB of RAM, but I don't...
The language homepage says "Arc is unfinished. It's missing things you'd need to solve some types of problems. [...] The first priority right now is the core language."
Perhaps parallelism is still pending. A Ctrl-F on the tutorial doesn't turn up any hits for "process", "thread", "parallel", or "concurrency".