The ongoing theme is that no-one has seen a white walker for thousands of years.
Without stating the obvious that this show is about ultimately having a war with with the un-dead, there seems to be nothing in the TV series that suggests why the white walkers have now decided to show their presence again.
They’re never far, you know. They won’t come out by day, not when that old sun’s shining, but don’t think that means they went away. Shadows never go away. Might be you don’t see them, but they’re always clinging to your heels.
- Tormund Giantsbane, A Dance with Dragons, Chapter 58, Jon XI
For what I can find, it seems that we are just going to have to wait and find out... Remember that this is a fantasy series and magic is definitely abound!
GRRM also likes to leave some gaps in knowledge and has even said we may not learn everything.
[Writer] How were the Other's defeated 8K years ago? In large numbers, they seem like they'd be invincible. Because their offensive against the Wall was checked, does that constitute a defeat? Was there a Human counterattack?
[GRRM] Well, it was thousands of years ago, so many of the facts are lost in the mists of time or have become legend. You will learn more in future volumes... but probably not everything, no...
[Writer] I must apologize for taking up valuable time to ask questions but this one is nagging me and I was hoping you would help. I was paging through Game of Thrones for the millionth time and came back to the tale that Old Nan was telling Bran about the Long Night and the last hero who went to find the Children of the Forest. Right at the scariest point of that story within a story Maester Luwin barged in, and the tale was never finished. Did the last hero ever find the Children of the Forest? What stopped the advance of the Others? Is there any relationship between the Children of the Forest and the Others? I realize we will most likely see more of the Others later on, but that old yarn is very interesting and I don't think Old Nan is still around to finish it. Any information would be greatly appreciated.
[GRRM] I'm afraid these are the sorts of questions that I will be answering in the books to come, rather than divulging via email.
[GRRM] It has been my intention from the start to gradually bring up the amount of magic in each successive volume of A Song of Ice and Fire, and that will continue. I will not rule out the possibility of a certain amount of "behind the scenes" magic, either. But while sorcerous events may impact on my characters, as with Renly or Lord Beric or Dany, their choices must ultimately remain their own.
[Writer] Are the Others just pure evil, or are we going to find out more about their motives later on?
[GRRM] Keep reading.
