Kangaroos carry their young in their pouch because they are a marsupial. Also, baby kangaroos (called Joeys) are very small when they're born. In fact, they are about the size of a lima bean. As you can imagine, a creature that small would have a difficult time surviving in the wild without some help from its mother.
In this case, the help is in the form of being carried in a pouch. While still living in their mother's pouch, they nurse, sleep, and do what babies do best: urinate and defecate. In case you're wondering, baby kangaroos don't jump or hop out of their mother's pouch. They sort of plop out onto the ground.
Kangaroos carry their young ones in a pouch called Marsupium. When a kangaroo is born, it crawls into one of the mother's teats. The mammary gland enlarges to form the pouch. Kangaroos have to carry their young ones in a pouch because the new born which is called Joey, are usually born very small, they are born at premature stages. Joey cannot survive on its own when it is born.
This pouch serves as a form of protection for the new born until they are grow to a level that they can survive on their own. Red kangaroo stay in the pouch for about 8 months and the grey kangaroo stay in the pouch for almost a year