You're absolutely right, not just mothers should be able to participate. However, the physical and emotional effects and trauma of childbirth are borne by women

You're absolutely right, not just mothers should be able to participate. However, the physical and emotional effects and trauma of childbirth are borne by women alone, so they're usually the party that needs to be incentivised. We're also operating within the social framework of Japan, which is deeply patriarchal.