Because they have a very high child mortality. When child mortality goes down, birthrates plummet.
Oman is a fairly developed country and they have twice as many kids per woman compared to poor Ukraine and almost-starving North Korea.
Cultural background is a huge influence in fertility. African immigrants in Europe have more children the poorer they are, the opposite of locals.
You’re right about the Haredi though. Congratulations, on a discussion of demographics across billions of people, you found one community of less than 2 million that buck the trend. Good for you.
Cultural background can have an influence, sure, but generally not enough to have a macro level effect. Hence the global trend.