It's because the bare TLD isn't setup to accept requests, but the `www` subdomain is (it's DNSed to a different IP):
$ curl -v --head https://nushell.sh/
* Trying 162.255.119.254...
* TCP_NODELAY set
* Connection failed
* connect to 162.255.119.254 port 443 failed: Connection refused
$ curl -v --head https://www.nushell.sh/
* Trying 185.199.108.153...
* TCP_NODELAY set
* Connected to www.nushell.sh (185.199.108.153) port 443 (#0)
Most hosts will alias or redirect away the www subdomain, but that's just a convenience. Of course technically foo.com and www.foo.com can have different DNS entries.