SIDS data shows a (relatively) high occurrence of kids dying by having parents roll over on them, or them getting entrapped/suffocated in bedding. In traditional cultures, this usually gets covered up.
Additionally, it’s much harder for the parents or kids to get consistent sleep (often already causing a lot of difficulties during this time), when toddler age kids are in the same area, due to differing schedules.
In the US, due to mainly socio-economic issues (need to migrate for work often results in families separated over long distances), it is relatively uncommon to have relatives living in the same household able to help with childcare too, especially late at night. So it puts a lot more load on the parents.
If not managed effectively, this can break the parents in ways no one likes to talk about in polite company.