`www` used to literally be a different host in a network (and in some cases, I'm sure still is) specifically designated for WWW traffic. Think of universities in the 90s which had their existing infrastructure and an Internet facing host on their primary domain and they want to add a web server. They may have had a firewall, probably no load balances, so routing port 80 around their primary host was much more complicated than just throwing up a new host and DNS entry.