What Is the Main Security Threat That Difficulty Adjustment Helps to Mitigate?

The main security threat mitigated is the "51% attack," where a single entity controls the majority of the network's hash rate. Difficulty adjustment ensures that the cost of acquiring and maintaining 51% of the hash power remains prohibitively expensive, even if the total hash rate fluctuates.

By making the mining process harder as more power joins, it maintains the integrity of the blockchain and the consensus rules.