What Economic Factors Might Prevent a Selfish Miner from Reaching the 51% Threshold?

The primary economic factor is the escalating cost of acquiring and maintaining the necessary hashrate. As a miner approaches 51%, the market price of mining hardware and electricity in their area will likely rise due to their increased demand.

Furthermore, the risk of a community-led defensive hard fork, which would destroy the value of their specialized hardware, acts as a significant economic deterrent.