Only Game Freak can provide you a definitive answer to this question. However, there is still some logical explanation available.
It's quite possible Game Freak wanted Kangaskhan to maintain its "mother" theme. It's exclusively female, and it carries a baby in its pouch. If the baby was its pre-evolution, it wouldn't be the same. It'd also be illogical if it its baby was its pre-evolution and it carried it in its pouch. A similar thing applies if a different Pokemon became its pre-evolution; it'd be really strange for a young Pokemon to instantly become a mother when it evolves, but then again, we have seen stranger things from Game Freak.
Kangaskhan's baby does come out in its Mega form though.