Not just HW and bandwidth, it's the software too.

File transfers were probably done via the XModem protocol, which did error checking only via a weak checksum, not even a CRC. Over a large file, the chances of corruption somewhere were dismaying.

And these here were text files, which should be hugely compressible. But at this point in history, compression algorithms were still in their infancy. I don't recall encountering ARC until the late 80s. So you'd be transmitting rather more than you needed to in the first place.