Excerpt from Buddhism in Plain Terms Volume 6, Chapter 43

Why has Bodhisattva said that people in the human realm can directly ascend to the heavens through cultivation? This is because this human realm is a transit point. It is arduous to practise Buddhism in the human realm; it will be easier to ascend to the heavens for those who put in more earnest efforts. 

Think about this, there are still other realms, how do those beings ascend to the heavens? For example, could those beings in the Animal realm ascend to the heavens? There is also the Asura realm. The beings there are joyful, carefree and in comfort. Do they still want to transcend the cycle of rebirth? No, they do not.

Why people in the human realm can directly ascend to the heavens through cultivation?