Why? Because it seems that Philippi was one of a select group of cities where the inhabitants had been granted a form of Roman citizenship.
لِذلِكَ حَمَلَتْ هذِهِ ٱلْعِبَارَةُ مَغْزًى خُصُوصِيًّا لِلْإِخْوَةِ فِي فِيلِبِّي .‏