How Does Decentralized Mining Mitigate the Risk of a 51% Attack?

Decentralized mining means that the total network hash rate is distributed among a very large number of independent miners and pools. When no single entity controls a significant portion of the hash rate, it becomes prohibitively expensive and logistically difficult for any one group to acquire the 51% needed for an attack.

A high degree of decentralization is key to the security of a PoW network.