Huge cultural stigmas and structures that reinforce stability family structures is likely at play. Similar to how other stats show Religious folks have a lower rate of divorce. Partially because of the forces encouraging to not divorce, both positively with huge support networks to keep couples together, and negatively with implicit shame for those who break up. And also partially that many who divorce may also leave said faith (eg self-selection Religiosity). This second component of course doesn't apply to immigrants.
Plus, a "no divorce" policy can also create systems where spousal/domestic abuse can run rampant and no recourse to escape. So it's not always the best metric to optimize for.