It's impossible to know for sure what happened, but if I had to guess I'd say it was something like this:
Martin wrote the first three books in a short time span. He was very into it. He decided to make it this grand long thing and made a bunch of public statements about how it would be seven books.
He started to struggle with the fourth book. He became more interested in the world of Westeros and exploring it than continuing the narrative he'd originally conceived. This is born out by the actual evidence at hand- the pacing of books and 4 and 5 is shitty, and bloated with lots of things that for as much as the stans will make excuses, are inconsequential to the overall narrative. See also: all the short stories and crap, and the World of Song of Ice and Fire or whatever book.
Martin has obviously put in more work towards non "core" ASoIaF Westrosi gibberish and other pursuits (being famous, other books, the tv show) than he has the main series in the past 16 or so years. The evidence just plainly affirms this.
So basically, if you ever read and enjoyed the Dunk and Egg stories or any of his other shit, you're to blame.
*stern gaze at PD*