How Does a Low Hash Rate Make a Cryptocurrency More Susceptible to a 51% Attack?

A low hash rate signifies that less computational power is dedicated to securing the network. This directly reduces the amount of hash rate an attacker needs to acquire to surpass 50% of the total.

Since the cost of renting or buying hash power is proportional to the amount needed, a low hash rate translates to a lower economic barrier for the attack. High hash rate networks are secured by a prohibitive cost to acquire majority control.