How Does Block Space Availability Directly Influence the Miner’s Zero-Fee Decision?

Block space availability is the primary factor influencing a miner's decision to include a non-whitelisted zero-fee transaction. If the block is full, the opportunity cost of including a zero-fee transaction is the foregone revenue from a fee-paying transaction, making the zero-fee inclusion irrational.

If, however, the block is not full (low-traffic environment), the opportunity cost is zero. In this case, a miner might include the zero-fee transaction simply to fill the block, as it does not displace any profitable transactions.