Why the pouch?
Kangaroo's
In the desert species, carrying the baby in the pouch is convenient for the female, who may travel many miles for fresh food and water. The youngster stands a greater chance of survival because it does not have to keep up with her and is tucked away from predators.
During prolonged drought, kangaroos stop breeding. In some species, a doe [the female] is able to delay the development of a fertilized egg inside her until an older joey dies or vacates the pouch.
This remarkable phenomenon occurs in the red kangaroo, the eastern grey kangaroo, the common wallaroo (euro), the brush-tailed bettong, and several of the larger wallabies. It has also been noted in the honey possum and some non-marsupial mammals such as bats and seals.2
Another incredible aspect is that the doe can determine the sex of her offspring. How she does this is unknown, but she tends to put off bearing males until she is older. Males move away after about two years, but females stay with their mothers longer and benefit from ongoing support.3
A doe is nearly always pregnant. From sexual maturity to death, she is rarely without three offspring — an embryo in the womb, a joey in her pouch, and a larger youngster at her heels.
The joey is born after a gestation period of about 35 days (depending on the species) and in the largest species is the size of a human thumb nail. In the smallest, it is only the size of a rice grain. Naked, blind and deaf, it must make its way unaided from the birth canal to the pouch.
All going well, the climb will take less than 10 minutes. The joey can survive only a few minutes unless it reaches the pouch and attaches to one of the four nipples. Once there, its mouth swells on the nipple so that it cannot be removed without injury. A ring of strong muscles, similar to human lips, seals off the opening to the pouch to protect the joey from bouncing out, and keeps the pouch waterproof if mother goes for a swim.4
After three months, the developed joey emerges from the pouch to make short trips in the outside world. However, it will return to the pouch to suckle and sleep until eight months old.

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animals are amazing. it just proves God's devotion to this world until we can be with Him in heavan. i was reading about Creation science a little bit ago, it's fascinating stuff. this site tells from a christian perspective how the animals dispersed, or at leaste some of there theories and speculations and of course biblical facts. read it, it's awesome.

http://christiananswers.net/q-aig/aig-c006.html