When networks were new, computers connected to each other using a shared trunk that you physically drilled into. It's a non-trivial problem to send data over a shared channel; it's very easy for two systems to clobber each other. A primitive, but somewhat effective mechanism is ALOHA (https://en.wikipedia.org/wiki/ALOHAnet), where multiple senders randomly try to send their message to a single receiver. The single receiver then repeats back any messages it successfully receives. In that way the sender is able to confirm its message got through -- an ack. After a certain amount of time with no ack, senders repeat their messages. As you can imagine, shorter packets are less likely to cause collisions.

Ethernet uses something similar, but is able to detect if someone else is using the wire, called carrier sense. A short packet of 1500 bytes reduced the likelihood of collisions.