First of all, God fulfills his promises of Isaiah 9:1–7, no matter how ancient they may be.
The second reason is very substantive. The north had first encountered the covenant curse of Deuteronomy 28:62–68 (see 2 Kings 15:27–31). They now got to share in the new light that Christ kindled! Isaiah had already promised this in those dark days. God does not forget this sinful covenant people either. He remains faithful!
13 And leaving Nazareth he went and lived in Capernaum by the sea, in the territory of Zebulun and Naphtali,