Why is it hard or challenging for some fathers to assume primary responsibility for their children?

It is always assumed the mother is always the one who is primarily responsible for them.
It can depend on the structure of the house and family. Does the wife stay at home or do both parents work?
The husband’s outlook on the family can be based on how a man sees their own father and what part he played in their family. Even though the husband may have this outlook, that does not make it right. Make sure you talk and come to an understanding.
Keep in mind, when your husband or wife keeps the kids, this is not doing your spouse a favor. The children are both your responsibility.