Spanning-Tree is based on an algorithm invented by Radia Perlman in 1985 and was published in a paper called ""An Algorithm for Distributed Computation of a Spanning Tree in an Extended LAN". The algorithm creates a loop-free topology by selecting a single root bridge and all other switches calculate a single least-cost path to the root.

Spanning-Tree prevents loops by breaking the looped physical topology into a logical loop-free one. This happens by placing each switch port in either a forwarding or a blocking state.

The short answer to this question is - Ethernet does not work in redundant topologies because of Ethernet loops also called layer-2 loops or broadcast storms. Without a technology that breaks the LAN with redundant links into a loop-free topology, BUM (broadcast, unknown-unicast, and multicast) frames would loop around indefinitely until a link or a device just fails.