
White Harbor was “built with the wealth that the Manderlys had brought with them from the Reach.” The previous occupants of the Wolf’s Den simply didn’t have the money to make the heavy up-front investments necessary to make the city secure (not only does White Harbor have the original Wolf’s Den and the much larger New Castle, but it also has city walls around both of those, and seawalls protecting the harbor, making it really, really hard to assault) and attractive to commerce (White Harbor might be small, but it’s got a double harbor with protective walls, jettys, a respectable shipbuilding industry, etc.)
Once those investments were made, they eventually more than paid for themselves, but it would have been hard for a lot of the smaller houses who held the Wolf’s Den after the Greystarks were brought down to make them in the first place. Moreover, the Manderlys had the advantage of making those investments at a time when the North wasn’t fighting wars against pirates and slavers on the one hand, and the Vale of Arryn on the other, so they had the opportunity to make their investments stick without seeing them burned to the ground.