Why is "isbn" considered a URN while http and sftp not?

I know http and sftp are protocols. But so is ISBN a an ID system for books. In the context of the web, I see both of them as "namespaces", aka URN. I understood URN as a "namespace" you give to URLs to preserve uniqueness. Like how android, angular and thymeleaf all declare their own namesapce on top of their respective XML layout files so you can do stuff like:

<p th:value="hello"></p>

Without fearing a naming collision by 3rd party software that may also name it's elements in the same manner. So in that case, why isn't http considered also a namespace? I see it as a namesapce.

EDIT: I gave it a little thought and I now start to doubt my idea of http being a URN/namespace since SSH and HTTP connections to the same locator URL offer two completely different results, so maybe protocols could be thought of as "connection meta data" not related to the locator. But if that is the case, which part of the common structure of http://www.example.com is the urn? ISBN is always given as an example but maybe 1% of us deal with ISBN enough for it to be a reference.