Science alert! I am absolutely fascinated by evolution, which is why I got an MS in it:

One of my favorite professors outlined an experiment where he compared a marsupial and a placental mammal of comparable sizes as to their energy expenditures. One of the main differences between placental mammals (normal type, no pouches/eggs) and marsupials (pouched) has been one of increasing gestation (mother --> baby via blood) time versus lactation time (mother --> baby via milk).

Milk was one of the big innovations of the mammals, but it surprisingly turns out to have been a rather bad idea compared to sustained internal development, which some other groups exhibit. It would seem advantageous at first, because it is remarkable in allowing the mother to rather passively nourish her young, but it (unsurprisingly) turns out to cost a bit of energy to turn your bloodborne nutrients into milk.

This is the reason why we only find the marsupials and the monotremes (eggs) in Australia. The evolution of the more efficient placental mammals occurred while Australia was isolated away, so it's the only place they weren't entirely outcompeted by their new kin. Mammals were predisposed to exploit certain available niches, and the similarity between competing placental-marsupial pairs always worked out in the placentals' favor.

Similarly, monotremes are even more inefficient than the marsupials in that they retain the ancestral reptilian egg-laying life history, whereby the young must develop in part without any active maternal support, and hereafter rely solely on milk as well. This is why there are only three monotreme species today, whereas at some point in the past, they comprised all of them. Evolution in action.

Or, we could just throw up our hands and say "God did it this way." Or, we could realize that evolution as we know it today has no need of divine intervention, and is sufficient to explain the real diversity of past and living species. Knowing this sort of thing is exciting, whereas a purely religious dead-end viewpoint seems much less so.