Just followed that and it's interesting (and a bit disappointing for those of us who breastfed):
Physiologic sleep studies of infants demonstrate that breastfed infants are more easily arousable than their formula-fed counterparts during sleep,54,88 which may explain a possible protective effect against SIDS. However, epidemiologic studies have not been consistent in demonstrating such a protective effect.16,18,31,49,89–98 Although some studies show a protective effect of breastfeeding on SIDS,18,98,99 others do not.31,49,91,96,97,100,101 In addition, a recent article has demonstrated that although breastfeeding is associated with decreased postneonatal deaths overall, it is not associated with a reduced risk of SIDS.102 Many of the case-control studies demonstrate a protective effect of breastfeeding against SIDS in univariate analysis but not when confounding factors are taken into account.31,49,91,96,97 These results suggest that factors associated with breastfeeding, rather than breastfeeding itself, are protective. One of these possible factors is nonsmoking, which is associated with a decreased incidence of SIDS and with both increased initiation and duration of breastfeeding.103–107 Although breastfeeding is beneficial and should be promoted for many reasons, the task force believes that the evidence is insufficient to recommend breastfeeding as a strategy to reduce SIDS.
I think it's drinking and bed sharing that is advised against, not room sharing? In which case it's to lower the risk of smothering. Smoking is a reason not to room share though, because it reduces the time in the day that the baby is exposed to your secondhand smoke.