Why Is Double-Spending Easier on a Blockchain with Low Hash Rate?

Double-spending is easier on a blockchain with a low hash rate because the cost and effort required to perform a 51% attack are significantly reduced. A low hash rate means an attacker needs less computing power to gain a majority (51%) control of the network's total mining power.

With 51% control, the attacker can create a longer, private chain containing the double-spend, then release it to the network, causing a re-org that invalidates the original, legitimate transaction.