Lack of 'determinism' in Ethernet bus
Etherenet is based on CSMA/CD,  which signifies carrier-sense multiple access with collision detection. It's not single master, any device can send and recieve independly, like multi master / multi slave, so that's why collisions happen. In such case the message sending is repeated, for both devices that had collided but each has it's own random timer, so that one takes over faster.

Determinism is more than just delivering a message from point A to B, but also at exact time.