Because Islam clearly tells that all believers are a brother to each other and one can't be a Muslim without loving the other. So if someone is burning people alive with white phosphorus, it hurts every single Muslim, whether they are in Belgium or China.
The Believers Are Like Brothers To One Another; So, Promote Peace And Reconciliation Among Them,
And Fear Allah, That You May Receive Mercy.
(Surah 49: Ayah 10)
He Has Weak Faith Until That Person
Loves For His Brother What He Loves For Himself.
Bukhari