Something I’ve not seen anyone else mention is the increase in processing power. It used to be that running a file server, mail transport, web server, etc were full-time jobs that a whole machine might be dedicated to. DNS allowed different addresses to resolve to different IP addresses which usually meant different machines, so each machine got it’s own domain. But different services run on different ports so it’s actually possible to run a web server (ports 80 and 443), email (25 and 465), file services (20 and 21), etc on a single machine with a single IP address - if the machine is capable enough. Even in situations where there are still multiple machines running different services, routers and reverse proxies have become more powerful too, so multiple services or even multiple versions of a single service can share a single external IP and domain name with the traffic being routed to the appropriate endpoint.

And, as others have said, saying “WWW” usually takes longer than saying the rest of the address so fuck that.