Even though several marsupial families lived in the Americas before the last ice age, opossums are the only ones still remaining. Thanks to their opportunistic omnivorous diet and high rate of reproduction, opossums have survived in their current form for millenia, even despite their extremely low encephalization quotient. While rote brain volume does not in and of itself determine intelligence of an animal, mammals with smaller encephalization quotients tend to be more specialized and quickly speciated when hardships are encountered (such as ice ages). Opossums in the Americas generally have an EQ around 1/5 that of the raccoons.
Didelphidae (Western hemisphere opossums) have very short lifespans, generally living less than two years in the wild, which is very unusual for a mammal of their size (up to the size of a large housecat). However, they can generally produce two successful litters of up to 13 young each in their short lives.
Australian opossums, while distantly related to those in the Americas, have furry tails, larger brains, and are much less urbanized. They also bear fewer young, live at least twice as long, and are less than half the size of the largest North American opossums.
Dictionnaire Universel d'Histoire Naturelle. Charles d'Orbigny, 1849.