Very helpful post by Daniel Miessler on URI vs. URL terminology.
Well, because we often use URIs in forms that don’t technically qualify as a URL. For example, you might be told that a file you need is located at files.hp.com. That’s a URI, not a URL—and that system might very well respond to many protocols over many ports.
If you go to http://files.hp.com you could conceivably get completely different content than if you go to ftp://files.hp.com. And this type of thing is only getting more common. Think of all the different services that live on the various Google domains.
So, if you use URI you’ll always be technically correct, and if you use URL you might not be. Finally, there is significant chatter around the term “URL” being—or becoming—deprecated. So URI is a fairly safe choice in terms of accuracy.