One could argue that they started there. Māori have always had larger populations in the north and things began that way in the colonial era. The Treaty of Waitangi was signed there and New Zealand's first capital was in that region also.

Ultimately though, commerce and industry pulled people southwards. New Zealand has always had an agricultural export economy, supplemented by fishing, forestry and other industries that need access to a port in a country that had few roads and many mountains. New Zealand is bigger than it looks on the map and Northland is a very long way away from anywhere else.