He has been writing other stuff for the past 8 years. That's the reason or he just doesn't want to write the book.
This is what he has been writing since Dance of the Dragons in 2011:
The Wit and Wisdom of Tyrion Lannister 2013 Quote collection from A Song of Ice and Fire The Princess and the Queen Novella A Song of Ice and Fire, prequels[140][141] The Rogue Prince 2014 The World of Ice & Fire Reference book The history of Westeros, with Elio M. García Jr. and Linda Antonsson The Ice Dragon Young adult illustrated novel Reworked version of the original novel published in 1980, illustrated by Luis Royo[142] A Knight of the Seven Kingdoms 2015 Collection compilation of the first three Tales of Dunk and Egg[143] The Sons of the Dragon 2017 Novella A Song of Ice and Fire, prequel[144][145] Fire & Blood 2018 Reference book & novella collection The history of House Targaryen