(The following from "I haven't thought about this for a long time" memory):
Technically, it is due to the "intermediate axis theorem" of dynamics. On any rigid body (a boat counts as a rigid body), there is an axis with minimum moment of inertia (ie: resistance to being turned) and an axis with maximum moment of inertia. It would be a strange monohull
where roll wasn't the lowest moment of inertia. Depending on the boat, either pitch
or yaw would be the highest. My guess is that with more modern boat designs (ie: most coastal cruisers), that yaw is the highest. If your boat is at anchor, and you want the most comfort, you want the bow into the seas. However, if pitch
is an intermediate axis, it will try to avoid pitching when it can yaw or roll. If there is little wind or current, then it will yaw until rolling gets initiated. The wave period can be close to the resonance frequency of rolling, so the boat will want to stay there -- beam to the sea, and rolling (sometimes extremely).