They have been poached just like all other species of rhino, but the civil wars in both DRC and Sudan have had a particularly devastating impact on the northern whites. Poaching initially increased during this time of civil unrest in the 1990s.
Despite this, the population managed to hold until 2003. Then there was another upsurge in poaching, sending the population plummeting. While the northerns fell, the southern white rhino grew, benefiting from a conservation strategy, mainly of translocation. Dr. Ian Player and a small group of men headed up efforts to move them out of harm's way, establishing new populations throughout southern Africa. The main population of these rhinos are the majority who are now in Kruger National Park.
Luck? Happenstance? Either way, it stands that one species is on the brink of extinction, while the other remains just steps away.