What Mechanism Ensures That Miners Prioritize Higher-Fee Transactions?

Miners are rational economic actors seeking to maximize their profit. They use specialized software to calculate the 'fee rate' (fee per unit of block space, e.g. satoshis per virtual byte).

The software then sorts the available transactions in the mempool by this fee rate, and the miner fills the block capacity with the highest-ranking transactions first. This purely economic mechanism drives prioritization.