Because they ordered the shutdown to brazilian internet providers. These internet providers company block the access for Whatsapp services. And they are more than happy to comply, as they hate Whatsapp for taking their SMS revenue. But they are in fact only obeying an justice order.

They are specially pissed off by whatsapp's free voice calls.

Ah. I wondered why the ISP's seemed so happy to comply.

Can't WhatsApp set up it's communications in a way that makes blocking them break a lot of other unrelated things?

They can simply block network traffic. They don't need WhatsApp to shut itself for Brazilians.

Because the shutdown is done by ISPs, but the request for user data can be ignored by WhatsApp.