The reason for the enforcement of a minimum length comes from the environment of the early days of Ethernet. In those early days the Ethernet interfaces needed to detect collisions. If a device at one end of the cable began to transmit a frame then it was necessary that it transmitted long enough so that its first bits reach all the way to the remote end of the cable before it ended transmitting, this would ensure that it would detect if another stations had begun to transmit and would enable all other stations on the cable to detect the activity.

To make an example think of an Ethernet cable with host A at one end and host  B at the other end. Host A begins to transmit and host B initiates transmission at exactly the same time. Host A has only a few bits to transmit and then stops. By the time the transmission from B gets to A then host A has ended its transmission and is idle. There is no detection that both hosts were transmitting and that their bits crossed each other part way through the cable.

HTH

Rick