Possibly because of varying sensitivities of the brainstem to serotonin:

The research team, led by doctors at Children’s Hospital Boston, compared brain tissue from 31 infants who died of SIDS from 1997 to 2005 with samples from 10 babies who had died of other causes. They focused on an area of the brainstem called the medulla, which regulates breathing, sleep-and-wake cycles and other vital functions.

They found, among other oddities, that cells in this region of SIDS babies’ brains were significantly less sensitive to serotonin than those in the other brains. The brainstem supports the autonomic nervous system, which helps rouse sleeping people if they are breathing too little oxygen, the authors said; and serotonin keeps the system responsive. The defects were particularly striking in male brains, which could account for boys’ higher risk of SIDS, they said.