Currently, the US Core profile for DocumentReference limits the attachments/content to 1..1. There are scenarios when a system might want to provide multiple. An example from Cerner is to provide the raw and a pdf version of the document that is more multi-platform display compatible. Other examples are documents that may have multiple attachments (Google).

It sounds like the history that led to the 1..1 decision today was taken from another document profile, and is likely a decision that we don't need to copy forward to US Core.