Why Is a 51% Attack More Difficult on Bitcoin than on Smaller PoW Chains?

A 51% attack on Bitcoin is prohibitively expensive because of the immense, decentralized scale of its network hashrate. Acquiring and maintaining the necessary specialized mining hardware (ASICs) and the associated electricity cost to consistently control over 50% of the global hashrate is economically infeasible for most entities.

Smaller PoW chains have much lower barriers to entry.