Look at the northern plain. A wide flat area with large rivers, very fertile, able to support large populations and produce vast wealth. The dependance on only two large river systems also means that the area is highly susceptible to being domianted by a single united polity, control strategic chokepoints on these rivers and the entire region must bend to your will.
Compare to the south, mountainous, far less fertile ground, cannot support populations as large, and is thus far less propserous. Furthermore it is easily fragmented as the defensible geography allows small groups to hold out against thier near peer neighbours isntead of beings subsmumed.
The north was simply far more powerful, and usually united agasint a fragmented and fractious south. Though southern geography gave them a marginal defensive advantage it was not enough against the sheer numbers and wealth of the north.
Combine this with the fact there was no peer power centre to contest the southern region with the Northern plain power centre, it was almost inevitable that the north would eventually conquer the south.