Why are marsupials primitive mammals?

A misnomer posed by some earlier poster is that it is like a reptile. The correct answer is, it is not considered the most primitive mammal. This was incorrectly presumed by 19th century scientists. It is actually uniquely evolved.

If you're attempting to find out the most primitive "modern" mammals, they could be argued to be the Soricidae, the shrews and the Talpidae, the moles, but any modern creature is simply not primitive. More basic, more complex, more or less evolutionary divergent from their ancestral norms, but not more primitive.