This is just my own understanding, but doesn't a webpage consist of a bunch of nodes, which can be combined in any way. And an html document is supposed to be a complete set of nodes, so a combination of those won't be a single document anymore.

Nodes can be addressed individually, but a document is the proportion for transmission containing also metadata. You can combined nodes as you like, but you can't really combined two already packed and annotated documents of nodes.

So I would say it is more due a semantic meaning. I think there was also the idea of requesting arbitrary sets of nodes, but that was never developed and with the shift away from a semantic document, it didn't make sense anymore.