The simplest reason: Mail servers have long had features which will send the mail client a substring of the text content without transferring the entire thing. Like the GMail inbox view, before you open any one message.

I suspect this is relevant because Quoted Printable was only a useful encoding for MIME types like text and HTML (the human readable email body), not binary (eg. Attachments, images, videos). Mail servers (if they want) can effectively treat the binary types as an opaque blob, while the text types can be read for more efficient transfer of message listings to the client.