We know why SM particles come in "whole generations": a mathematical consistency condition known as anomaly cancellation [0]. We also know that with only 2 generations we cannot get violation of charge-parity symmetry [1] via the Cabibo angle [2] alone; to get CP violation we need at least a 3x3 CKM matrix [2]. CP violation is one of the Sakharov conditions [3] that is required to get matter/antimatter asymmetry from early universe dynamics.
This is not an a priori reason, per se, but it does waggle its eyebrows up and down in a sort of anthropic way.
