Two thoughts
Jamie and Brienne are going to hook up
More importantly, how did they get the wight to kings landing past the wall since magic prevented the creatures from passing? Yes, it came on a boat but magic is at play. If that's the case, wights and white walkers could've gotten on boats centuries ago.
Maybe bringing that one wight past the wall weakened the magic of the wall.
Jamie and Brienne are going to hook up
More importantly, how did they get the wight to kings landing past the wall since magic prevented the creatures from passing? Yes, it came on a boat but magic is at play. If that's the case, wights and white walkers could've gotten on boats centuries ago.
Maybe bringing that one wight past the wall weakened the magic of the wall.