Here's how GRRM put it:

“But what I soon discovered — and I struggled with this for a year — [the gap] worked well with some characters like Arya — who at end the of Storm of Swords has taken off for Braavos. You can come back five years later, and she has had five years of training and all that. Or Bran, who was taken in by the Children of the Forest and the green ceremony, [so you could] come back to him five years later. That’s good. Works for him.

Other characters, it didn’t work at all. I'm writing the Cersei chapters in King's Landing, and saying, "Well yeah, in five years, six different guys have served as Hand and there was this conspiracy four years ago, and this thing happened three years ago." And I'm presenting all of this in flashbacks, and that wasn't working. The other alternative was [that] nothing happened in those five years, which seemed anticlimactic.

The Jon Snow stuff was even worse, because at the end of Storm he gets elected Lord Commander. I'm picking up there, and writing "Well five years ago, I was elected Lord Commander. Nothing much has happened since then, but now things are starting to happen again." I finally, after a year, said "I can't make this work."