Why Is Collision Resistance Generally Considered a Stronger Requirement than Preimage Resistance?

Collision resistance is stronger because it requires the hash function to resist finding any two inputs that map to the same output. Preimage resistance only requires resisting finding the input for a specific output.

Due to the Birthday Paradox, a collision can often be found much faster than a preimage (in O(2^(n/2)) time versus O(2^n) time for an n-bit hash). Therefore, a hash function must have a sufficiently large output size to resist collision attacks.