DyDx Liquidations succeed because backrunners are quick to exploit the MEV.
To put simply, transaction A is broadcasted with a slightly lower gas price than already pending transaction B so that A gets mined right after B in the same block. (eg. to execute a DyDx liquidation after a price oracle update that triggers a DyDx loan to go under the required collateralization ratio).
Fairly interesting! To learn more about it: