Game of Thrones has both started and ended its story, with The Winds of Winter delayed a number of times.
The sixth and penultimate book in Martin's series, its release date has become the subject of intense scrutiny. It was previously reported that GRRM's itted he's "struggling" to write TWOW, but why?
Firstly, Martin's life has changed a lot since the last ASOIAF book was released. Game of Thrones became the biggest TV show in the world, thrusting him further into the spotlight and providing both increased opportunities and distractions. Since then he's worked as a writer, executive producer, and co-creator within the TV industry, and not just for Game of Thrones. He's also written other books, with Fire & Blood expanding from part of The World of Ice and Fire into a two-volume tome itself, the first half of which released last year. Martin has many hats to wear, both literally and figuratively, which have taken him away from The Winds of Winter.
However, there is a bigger reason than simply being busy, and it's to do with the story Martin has created. He originally planned to write a trilogy, but as the plot kept growing, so too did the number of books. What started out as a relatively tight narrative is now a sprawling fantasy series with more characters than perhaps even he could name, and a story that has kept on stretching further apart rather than coalescing back together. In A Game of Thrones, there were eight point-of-view characters (excluding the prologue). In The Winds of Winter, there could be 15-20, meaning the story has roughly doubled from where it started.
There are so many disparate plot threads that it's no wonder Martin, who its he doesn't plan too far ahead but instead sees where the characters take him, is struggling to bring it to a close with the final two books. We saw how ending will be different in the books to Game of Thrones, with many more characters still alive and storylines to address, and it's clear Martin has no interest in racing to the finish line as David Benioff & D.B. Weiss did.
At the same time, it's become increasingly difficult for him to write his way out of the narrative dead-ends, distractions, and diversions he's taken. The Meereenese Knot is the most well-known example (relating to the convergence of characters in Slaver's Bay), but we can see it in elements such as Young Griff, Dorne, and the Ironborn plots, all of which are interesting in their own right, but mean the story has expanded almost beyond control.
On top of that, he's also faced with the Others becoming increasingly important. That means not only figuring out the mystery behind them, but making characters face them without this story - which was a response to so much of the post-Tolkien literature - becoming overly conventional fantasy, and avoid succumbing to the tropes it's always subverted. Martin has to try to bring this epic story together and find a way of uniting so many characters, but also set up a key storyline that is more good vs evil. It's an almost impossible balancing act, and no surprise The Winds of Winter is taking GRRM so long to write.