There could be any number of reasons. He could be banished or on a mission away from the migrating herd. He could be the type of man who felt he only needed to, erm...contribute to making the egg and then flounce off. Or he could be still with the herd and doing father duties for Ali but the story makes the parents such a low priority they only felt like showing one of them.
Not that I mind too much. Mamas for the win!