Because people realised you can't properly raise more than 6 kids.
nikkas in other countries think shyts sweet because out of their 15 kids, three are in a militia, one got killed in a drone strike, two are in prison, one joined a seminary, two went missing in adolescence, one died of malaria, and the other five are the "success stories" because they're all in school. Meanwhile one is pregnant from a 35 year old man, and another is on the verge of suicide.
When have you ever seen a stable family of more than 6 kids? Honestly stable.