What Is the Primary Incentive for a Bitcoin Miner to Include a Transaction in a Block?

The primary incentive is the total reward for the block. This reward consists of two parts: the newly minted coins (the block subsidy) and the sum of all transaction fees included in that block.

As the block subsidy halves over time, transaction fees become an increasingly important part of the miner's revenue. Miners prioritize transactions with higher fees per byte to maximize their profit from the limited block space.