Why are marsupials a thing?

The pouch of these animals serves to accommodate the undeveloped young that comes out of the womb at birth, and that stays there until it is fully developed. Zoologists and paleontologists have debated for years why and how the marsupium evolved.
Granted that it is a more primitive and older solution than placenta, it seems that the answer comes from the nature of the Australian continent and the advantages it offers there.
Marsupials were once scattered across the Earth, especially in South America and Australia, animals such as the thylakosmilus, the marsupial version of the saber-toothed tiger:

Or cladosites (marsupial otters):

Or South American Prothylacinus (marsupial wolf):

All these and many others were common animals in South America before it rejoined North America. Fossils of now-extinct placental mammals have been found in Australia, a sign that they were present on the continent long before humans arrived.