Most diverse and numerous in Australia, where there is negligible
competition with eutherians
Is the competitive disadvantage the result of their mode of reproduction?
Maybe. Their mode of reproduction limits the environments in which they
can live.
Maybe not. Other factors include:
Cerebral cortex of marsupials is smaller and develops more slowly
Learning and behavioral flexibility is less developed in marsupials.
Behavior is less diverse.
They have a small number of chromosomes, which may make them less
evolutionarily flexible.
Their reproductive rate is lower.
An alternate view is that marsupials are not necessarily at a competitive disadvantage in all circumstances. Possible advantages of a marsupial reproductive stratgey include:
Low energy requirements, spread out of a long period of time